Read a tire code left to right. In our example, P225/60R17 98H, the P is the type, 225 the width in millimeters, 60 the sidewall height as a percentage of the width, R radial construction, 17 the wheel diameter in inches, 98 the load index and H the speed symbol. Never fit a tire whose load index is lower than the one your car was built with. The size your car was built with is on the tire placard, usually on the driver's door pillar, and in the owner's manual, not on the tires you happen to have: they may not be the original ones. The last four digits of the DOT number give the week and year the tire was made, so 0109 is the first full calendar week of 2009. What you may change: the tire makers we read allow a higher load index or a higher speed symbol than the placard's, never a lower load index, and a lower speed symbol only where the maker accepts it (mostly winter tires), with the car kept below the tire's top speed. On an all-wheel-drive car, read your owner's manual before replacing fewer than four tires. UTQG treadwear compares tires within one maker's line, not mileage; Bridgestone says not to compare it across brands. Fill to the cold pressure on your car's tire placard, never to the maximum printed on the tire's sidewall. Check any size that differs from the placard with a tire professional and with your owner's manual before you buy; the makers we read do not agree on how much freedom you have.
We make PulsCar, an app that works with a car's diagnostic port, and a free engine sound check. We sell no tires and this page has no affiliate links: nothing here earns us money when you pick a size, a brand or a shop. Everything on it is what other people print: the federal regulations on tire marking and grading, two US tire trade groups, the tire makers' own guides, and car makers' owner's manuals and support pages, read on 5 November 2026 and saved as text.
The question is narrow on purpose. The code on a tire looks like a license plate: what does each piece mean, where do you find the size your car needs, and when can that size be changed? The page does not tell you which tires to buy or why a tire pressure light is on, and it sets no replacement date: when to replace tires covers that. Our guides on the tire pressure light, wheel alignment cost, a car that pulls to one side and a car that shakes at highway speed cover those.
The whole code in one example: P225/60R17 98H
Most of the tires on US cars and light trucks carry a size line that looks like this. The example below is ours, not a particular tire: it is built to show every part once, and the arithmetic in it is ours too (one inch is 25.4 millimeters). The breakdown itself matches what Michelin, Pirelli, General Tire, the Tire Industry Association and car makers such as Hyundai print for their own examples.
One more point before the details. Federal standard FMVSS 139, 49 CFR 571.139, requires a new passenger or light-truck radial tire to carry the symbol DOT, the tire size designation, the maximum permissible inflation pressure, the maximum load rating (plus the load range letter for LT tires), the cord materials, the number of plies, the word tubeless or tube type, the word radial, the maker's name or brand, and a tire identification number. A speed symbol is not on that list, and the load index is not on it in that form; the placard rule in 49 CFR 571.110 makes the load index and speed rating an option for the car maker to print beside the tire size. Tire makers print both anyway; in these two texts they are a convention, not a requirement we could find.
The letters before the numbers: P, LT, ST, T, or nothing
The first thing you may see in front of the width is a letter. Several sources define them, and they agree.
P means a passenger tire. Michelin's page says a P prefix "identifies a passenger tire built for cars, minivans, SUVs, and light-duty trucks used mainly for carrying people." Bridgestone says most tires will have the letter P, short for P-metric. LT means light truck. Michelin says an LT prefix identifies a tire with "a stiffer carcass and higher load capacity", and Toyo puts the use more bluntly: "Tires with a “LT” are for light truck or SUV use only." ST is a special trailer tire, and T is a temporary spare; Bridgestone says ST is used on trailers while T denotes a temporary tire, like a donut spare you might find in the trunk.
No letter at all is common too, and the Tire Industry Association explains it: "The absence of a letter at the beginning of the size description indicates that the tire is Euro-Metric." It then adds the part that matters for a buyer: "The main difference is that Euro-Metric tires may have different load carrying capacities than their P-Metric counterparts." Yokohama says something close: Euro-metric sizes are dimensionally equivalent to P-metric sizes but typically differ subtly in load-carrying capabilities. We come back to why this matters when the page reaches substitutions.
The letter is also the first place a swap can go wrong. Michelin warns that if a truck or SUV's factory tires are LT-rated, fitting P-metric tires of the same size will leave you short on load capacity, which it calls a safety issue. Toyo's tire safety document is firmer still: "NEVER mix P-metric or European Metric passenger tires with light truck sized tires on the same vehicle."
Width, aspect ratio and wheel diameter: the three numbers that must fit
After the letter come three numbers in a fixed order.
The three-digit width. In the example, 225. Yokohama defines it as the section width in millimeters, adding that the actual tire may not match perfectly, but it is close. Toyo's wording is more exact: "This is the width of an inflated tire in millimeters at its widest point from sidewall to sidewall." Pirelli says the same in other words. The width is a nominal figure, and the measured width of a real tire can differ slightly from it.
The two-digit aspect ratio. In the example, 60. It is not a height; it is a percentage. The Tire Industry Association gives the arithmetic in its own example: "The 60 refers to the aspect ratio, which is the nominal sidewall height reflected as a percentage of section width. So the sidewall height on this tire is 60% of 225 or approximately 135 millimeters." Toyo adds the rule of thumb: "The lower the aspect ratio, the shorter the sidewall and the wider the shoulder-to-shoulder width." A 60 is a taller sidewall than a 45.
The wheel diameter. In the example, 17, in inches, even though the width is in millimeters. The Tire Industry Association says it represents the diameter of the beads, so the tire is designed to fit a 17-inch rim. The diameter is a hard limit: a 17-inch tire does not fit a 16-inch or an 18-inch wheel.
Put together, those three numbers fix the overall height of the tire, and that is what the substitution rules later in this page care about. Using our arithmetic:
The overall diameter is the wheel diameter plus two sidewalls, which is why the figure changes when either the aspect ratio or the wheel size changes. General Tire's page gives the same recipe in words (sidewall height from the width and aspect ratio, double it, add the wheel diameter); the numbers in the box above are our application of it to this example, in millimeters.
The construction letter. Between the aspect ratio and the wheel diameter sits one letter, R in the example. Hyundai's manual calls it the tire construction code (radial). The Tire Industry Association adds: "The “R” stands for radial construction, which is the industry standard for passenger car and light truck tires." A B, X or a dash in its place indicates bias-ply construction. GM's manual lists the letters R for radial, D for diagonal or bias ply and B for belted-bias. The reason it matters: Subaru's manual says to "Use only radial tires. Do not use radial tires together with belted bias tires and/or bias-ply tires," and Mazda's 2020 Mazda3 manual says to use the same type of tires on all four wheels except for the limited use of the temporary spare.
The load index: a code for a weight, not the weight
The two characters after the diameter, 98 in the example, are the load index. The common mistake is to read them as pounds, and the makers say so in nearly the same words. General Tire: "Load index 85 indicates a carrying capacity of 1,135 pounds, not 85 pounds or 850 pounds or 8,500 pounds." BFGoodrich: it "doesn't mean 97 pounds, because it's actually an assigned value that corresponds with its “actual” load capacity found on a load index chart."
The index is a lookup. The Tire Industry Association describes it as a code for the maximum load that can be carried at the speed indicated by the speed symbol. Pirelli and Nitto both print the same data point: a load index of 96 means 1,565 pounds, and both say this is at maximum inflation pressure. BFGoodrich says the indexes on passenger-car and light-truck tires typically run from 70 to 110, and General Tire says 70 to 126.
The values are from Hankook's US page, which prints a kilogram and pound conversion; BFGoodrich's 97, 98 and 99 (1,609, 1,653 and 1,709 pounds) and the 96 figure from Pirelli and Nitto agree with it. In the example, 98 is 1,653 pounds for one tire. We stopped at 91 to 100 because the other ranges of that table did not agree with other makers' figures; for an index outside this range, use the tire maker's chart or ask the shop.
Three cautions the sources add. A higher index does not raise your car's limit. Michelin: "Installing a tire with a higher load rating is permitted. However, this does not increase the vehicle’s maximum allowable load, which remains limited by the vehicle’s axle load ratings and manufacturer specifications." An equal index is not always equal capacity. Toyo's guide to load and inflation tables: "An equal or greater load index does not always correspond to equal or greater load capacity at all inflation pressure settings, particularly when comparing P-metric and Euro-metric passenger car tires." You may not find an index on every tire. Nissan's manual: "You may not find this information on all tires because it is not required by law." The placard and the manual, not the sidewall alone, are the safer source when you replace a tire.
The one rule. Michelin: "Never fit tires with a lower load index than the factory specification." The Tire Industry Association: "Never install a tire with a load index that is lower than the original equipment tire listed on the placard or in the owner’s manual." Toyo: "Replacement tires must have a load index equal to or greater than the original equipment (OE) specification recommended by the vehicle manufacturer." The rule in one sentence: never fit a tire whose load index is lower than the one your car was built with.
How does reading a tire's load index relate to PulsCar, and does PulsCar know which tires my car needs? PulsCar does not sell tires, and this page has no affiliate links. PulsCar makes an app that works with a car's diagnostic port and a free engine sound check built on a 30-second clip. Neither one reads a tire's sidewall, and a sound clip cannot tell you a load index. The load index that matters for your car is the one on your placard and in your owner's manual, which the Tire Industry Association and Michelin both say to follow, and the index on the tire in your hand has to be at least as high. Check it on the tire before you pay, because Nissan's manual says it is not on every tire.
The speed symbol: a rating, not a speed to drive
The last character, H in the example, is the speed symbol. Michelin lists the common ones: "H = 130 mph, V = 149 mph, W = 168 mph, Y = 186 mph." The cleanest complete row for S to Y is in the Hyundai and Kia manuals; Hankook's, General Tire's and Toyo's pages add the symbols below S, and Hankook's the ones above Y.
What the symbol is not. The Tire Industry Association: "The Speed Symbol, or V in this example, indicates the maximum speed for the tire, but it is more like a performance rating that reflects the handling characteristics after it is installed on the vehicle." Toyo: "Tire speed ratings do not imply that a vehicle can be safely driven at the maximum speed for which the tire is rated, particularly under adverse road and weather conditions or if the vehicle has unusual characteristics." Nissan's manual: "Never exceed the maximum speed rating of the tire."
A lower symbol: the makers differ, so read each. Michelin: "Tires with a higher speed rating than originally specified may be installed." And: "Using tires with a lower speed rating is not recommended unless the tires are winter-capable, such as Winter or All-Season tires marked with the 3-Peak Mountain Snowflake (3PMSF) symbol." Toyo, asked about a lower speed-rated tire: "Generally, no. It is recommended that replacement tires meet or exceed the speed rating specified by the vehicle manufacturer." BFGoodrich: you can always increase the rating, "but can never decrease it without reducing the vehicle top speed to that of the lower speed rating selected." Nitto: "Nitto does not recommend mixing different speed rated tires on any vehicle." (That is about mixing on one car, not about a single lower choice.) BFGoodrich and Nissan's 2019 Titan manual also accept a lower rating with a speed cap, and GM's 2019 Tahoe and Suburban manual says that if winter tires with a lower speed rating are chosen, "never exceed the tire's maximum speed capability." Goodyear's UK site (its US pages did not load as text for us) allows lower-rated all-season or winter tyres so long as a speed sticker is in the driver's view and the vehicle is not driven faster, and adds to check your vehicle manufacturer's documentation first.
None of the pages we saved says a lower symbol is safe in general. If your placard carries a speed rating, match it or exceed it. For a winter set that goes lower, ask the tire shop and read your owner's manual first.
Other marks on the sidewall: XL, load range, max pressure, M+S and the snowflake
XL, Extra Load, Reinforced. Yokohama says passenger tires are marked light load (LL), standard load (SL) or extra load (XL), and that light-load passenger tires have a maximum load pressure of 35 or 36 psi while extra-load ones have 41 or 42 psi. Michelin: "If your vehicle's factory tires are marked XL, replacements must also be XL-rated — a standard load tire of the same size won't have the capacity needed."
Load range (LT tires). Light-truck tires add a letter. Yokohama says the range runs from B to F and that most tires on the market are C, D and E; Pirelli says the letter "more generally specifies the toughness of the tire and the amount of air pressure it can hold."
Maximum load and maximum pressure. Under FMVSS 139 each tire carries both on its sidewall. The key line is what they are not. Michelin: "Sidewall markings such as MAX LOAD and MAX PRESS indicate the maximum load and maximum inflation pressure the tire is designed to withstand. These values are not the recommended operating pressure for your vehicle." Yokohama says the recommended pressure can always be found in the placard in the driver's door panel. In one sentence: fill to the cold pressure on your car's tire placard, never to the maximum printed on the tire's sidewall. The tire pressure light guide explains how to measure and why a gauge beats the dashboard.
M+S, and the mountain with a snowflake. Two marks, two meanings. Michelin: M+S "indicates the tire is designed for mud and snow conditions based on the manufacturer’s criteria, but does not require standardized winter performance testing. Only tires marked with the 3PMSF symbol provide verified winter performance." Under FMVSS 139 the pictograph (three peaks, the middle tallest, with a six-sided snowflake inside) may be shown on a tire that meets the standard's snow-tire definition, a snow-traction test against a reference tire. Whether an all-season tire is enough for your winter is a separate question; our all-season tires guide takes it up.
The DOT number and the date code: when the tire was made
Find the letters DOT on the sidewall. After them comes a string of letters and digits, the Tire Identification Number (TIN), and its last four digits are the date. 49 CFR 574.5 sets the format: a new tire's number has 13 symbols, a three-symbol plant code, a six-symbol manufacturer's code and a four-digit date code. The regulation's wording for the date:
"The date code, consisting of four numerical symbols, is the final group. The date code must identify the week and year of manufacture." Its example: "0109 means the tire was manufactured in the first full calendar week of 2009, or the week beginning on Sunday, January 4, 2009, and ending on Saturday, January 10, 2009."
Michelin prints the same lesson: "For example, 2524 means the 25th week of 2024." The US Tire Manufacturers Association says the digits are the week of manufacture (01 through 52 or 53) and the year "for any tire made in 2000 or later."
Two practical points. The date may be on only one side: Toyo says the partial number on the opposite sidewall "does not show the week and year that the tire was manufactured." If you cannot find the date, look at the other side of the tire. And Pirelli says the code is usually printed in smaller letters near the bead; in poor light a flashlight helps, which is our tip. Tires made before 13 April 2025 may carry an older layout, because the regulation replaced the two-symbol plant code with a three-symbol one and allowed the old code until that date; the last four digits are the date in both.
What the regulation calls the DOT symbol. "The DOT symbol constitutes a certification that the marked tire conforms to an applicable Federal Motor Vehicle Safety Standard." It is the maker's certification of compliance, not a grade, and it says nothing about age or quality beyond that.
How old is too old: what the makers print about tire age, side by side
The date code answers when the tire was made. It does not say when to retire it, and the sources do not agree on a number. We list them side by side and do not average them. This page only reads the date and prints what the sources say; the decision is covered in when to replace tires.
The trade groups are careful. The Tire Industry Association's pages we read carry no age number. The US Tire Manufacturers Association: "Service and storage conditions vary widely, so accurately predicting the actual serviceable life of any specific tire based on simple calendar age alone is not possible." It adds that a tire or vehicle manufacturer's recommendation "is not a guarantee or indication of the minimum serviceable life for your tire." Michelin: "Tire age alone does not determine whether a tire is usable; tread wear and overall condition are also critical factors." Your owner's manual is the first stop for a number, because the car maker's figure is the one written for your car, and car makers and tire makers differ, as the table shows.
A new tire has a date code too, so ask the shop to show it before the tire goes on the car. A spare ages too: the Honda, Hyundai, Kia, GM, Mazda and Nissan documents we read include it in their statements.
How old can a tire be when I buy it, and does PulsCar check tire dates? The sources we read give no limit for a new tire's shelf age, so we state none. What they give is where to look: the last four digits of the DOT number, read as week then year, on the outboard sidewall of a tire made on or after 1 September 2009. PulsCar does not check tire dates and does not sell tires; its app works with a car's diagnostic port and its free engine sound check listens to an engine. For a tire's age, ask the shop to show you the date code before the tire goes on, and look at the spare in your trunk yourself, because the Honda, Hyundai, Kia, GM, Mazda and Nissan documents we read include the spare in their age statements.
UTQG: treadwear, traction and temperature, and what they promise
UTQG, the Uniform Tire Quality Grading, comes from 49 CFR 575.104. It applies to "new pneumatic tires for use on passenger cars" and not to deep tread or winter-type snow tires, space-saver or temporary use spare tires, tires with nominal rim diameters of 12 inches or less, or limited production tires. The grades are molded into the sidewall between the tread shoulder and the maximum section width. The three grades, as the regulation defines them:
Treadwear is a comparison, not a mileage. The regulation's explanatory text, which the rule requires car makers, tire makers and brand owners to pass on: "For example, a tire graded 150 would wear one and one-half (1 1/2) times as well on the government course as a tire graded 100. The relative performance of tires depends upon the actual conditions of their use, however, and may depart significantly from the norm due to variations in driving habits, service practices and differences in road characteristics and climate." (The grades themselves are multiples of 20; 150 is the regulation's illustration.) The Tire Industry Association: "the Treadwear rating cannot project the actual tread mileage of a tire nor can it be accurately used to compare the projected tread life of one brand against another." Bridgestone: the grades "are assigned by each tire maker to its own products. They may be used to make comparisons within a tire maker’s product line, but because of variations in testing procedures, they should not be used to compare tires from one company with those made by another company." Yokohama's page goes further, saying the marking "demonstrates how long a tire can last"; the regulation, the trade group and Bridgestone are more careful, and we follow them.
Traction is straight-ahead wet braking. The regulation: "The traction grades, from highest to lowest, are AA, A, B, and C. Those grades represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. A tire marked C may have poor traction performance." Then the warning most shoppers never see: "The traction grade assigned to this tire is based on straight-ahead braking traction tests, and does not include acceleration, cornering, hydroplaning, or peak traction characteristics."
Temperature is heat resistance in a laboratory. "The temperature grades are A (the highest), B, and C, representing the tire's resistance to the generation of heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel." Grade C is the minimum every passenger-car tire must meet under Federal Motor Vehicle Safety Standard 109. Its warning: "Excessive speed, underinflation, or excessive loading, either separately or in combination, can cause heat buildup and possible tire failure."
What UTQG cannot do. It cannot tell you the size, the load index or the speed symbol, and it does not apply to winter-type snow tires.
Where to find the size your car needs: the placard first, then the manual
The tire you are looking at is the wrong place to start. It tells you what is on the car today, which is not always what the car was built for. The size the car maker chose is printed on a label, and federal rules say where.
The tire placard. FMVSS 110, 49 CFR 571.110, requires each vehicle (except trailers and incomplete vehicles) to carry a placard "permanently affixed to the driver's side B-pillar." If that spot cannot hold a legible placard, the standard moves it to the rear edge of the driver's door and then to a surface next to the driver's seating position; a car with no driver's-side B-pillar and two opposite-opening doors on that side uses the forward edge of the rear door. The placard shows the vehicle's capacity weight, its seating capacity, the recommended cold inflation pressure for front, rear and spare, and the size of the tires installed at first purchase, under the heading "Tire and Loading Information." A maker may put the size and pressure on a separate tire inflation pressure label instead. A load index and speed rating next to the size are optional for the maker to print, so your placard may not show them.
The certification label. A second label, the certification label, goes on the hinge pillar, door-latch post or door edge next to the driver's seat. For passenger cars sold with optional tire sizes, 49 CFR 567.4 lets the label list several sets of weight ratings, each followed by "with __tires," naming the sizes, so a rating can be tied to a tire size.
The owner's manual and the car makers' wording. Car makers say the same thing in their own words. GM's manual: "The Tire and Loading Information label indicates the original equipment tires on the vehicle." Honda: "The label attached to the driver’s doorjamb provides necessary tire and loading information." Subaru: "The tire placard is located on the driver’s door pillar." Hyundai puts its tire label on the driver's side center pillar. Kia's manual defines the vehicle placard as "A label permanently attached to a vehicle showing the original equipment tire size and recommended inflation pressure." Tire makers and trade groups add other places. The US Tire Manufacturers Association: "Fortunately, your tire specifications travel with you—usually on a label inside the driver’s door, in the glove box or in the fuel door." Firestone lists the driver's door jamb, the inside of the glove compartment door and the gas tank hatch, and Toyo adds the trunk lid.
Why the placard beats the tire in your hand. The standard's wording is the tell: the size on the placard is for the tires installed "at the time of the first purchase for purposes other than resale." BFGoodrich says your correct size is printed in two places, the sidewall of the tires on your vehicle now and the placard inside the driver's door. If the two differ, a previous owner or a shop may have changed the tires, or the car may legitimately list different front and rear sizes (BFGoodrich notes that some performance cars list them separately); the placard and the manual are the car maker's answer. Some makers go further than size: GM's manual says vehicles with a tire pressure monitoring system "could give an inaccurate low-pressure warning if non-TPC Spec rated tires are installed," and Nissan's manual says tires not originally specified could affect the low tire pressure warning system. Our tire pressure light guide covers that system.
Which tire size does my car need, and can PulsCar tell me? Your car's required size is the one on its placard, usually on the driver's door pillar, and in the owner's manual; the load index on a replacement tire must not be lower than the car maker lists, and the speed symbol should match or exceed it, though a few tire makers, and GM for winter tires, allow a lower one on winter or all-season tires, with the car then kept below the tire's top speed. PulsCar cannot tell you a tire size: it does not sell tires, this page carries no affiliate links, and its free engine sound check listens to the engine, not the tires. Photograph the placard and take it to the tire shop; it lists the spare's size under the word spare.
Which changes the makers allow, and which they warn against
Everything so far was about reading. This section is about changing, and it is where the sources split. We separate them into three layers so you can see who says what.
Layer one: the baseline. The US Tire Manufacturers Association says a replacement tire should "Be the same size, load index and speed rating designation as recommended by the vehicle or tire manufacturer" and should "Not be of a smaller size or load-carrying capacity than what was originally specified. That could be dangerous."
Layer two: the car makers' manuals we read keep the size or warn against changing it. Hyundai: "ALWAYS replace tires with the same size, type, construction and tread pattern as each tire that was originally supplied with this vehicle." Honda's tire sheet: "Using tires of a different size or construction can cause certain vehicle systems such as ABS and Vehicle Stability Assist (VSA) to work incorrectly." Nissan: "When replacing a tire, use the same size, tread design, speed rating and load carrying capacity as originally equipped." Subaru's manual says a non-specified size detracts from controllability, braking performance and speedometer accuracy. Mazda's 2020 Mazda3 manual calls any other size dangerous. GM warns that a different size may affect braking, handling, stability and resistance to rollover, and electronic systems such as antilock brakes and stability control.
Layer three: tire makers describe how a change can be done. BFGoodrich: yes, a different size is possible, "but all vehicle manufacturer specifications must still be met, including load capacity, speed rating, and overall rolling diameter within a reasonable tolerance." Toyo keeps the car maker on top: "ALWAYS refer to and follow the vehicle manufacturer's replacement tire recommendations. In some cases, a vehicle manufacturer may specifically advise against the application of replacement tires that are not the original size or type." The Tire Industry Association says custom tire and wheel packages that differ from the placard require a tire professional.
So the makers agree on a small core and disagree about what lies beyond it. The table shows the core and the disputed parts.
For the disputed part, the advice is plain: check any size that differs from the placard with a tire professional and with your owner's manual before you buy. If the manual and a tire seller disagree, the manual is the car maker's word about your car.
Plus sizing: how a bigger wheel changes the tire, with the arithmetic
Plus sizing means a larger wheel with a lower-profile tire. BFGoodrich: "Tire upgrading, also called plus-sizing, is the practice of replacing your original equipment wheels with wider, larger diameter wheels and correspondingly lower profile tires." Toyo defines the first step: Plus One means the wheel is one inch larger, with a tire of increased section width and lower aspect ratio. Its figures for that step, as a general rule of thumb, are "Increases section width by 10 mm. Decreases aspect ratio by 10 points. Increases rim diameter by 1 inch."
The reason aspect ratio drops when the wheel grows is arithmetic: the tire's overall height has to stay about the same. BFGoodrich gives the guideline: "keep overall diameter within about 3 percent of original. Beyond that, speedometer and odometer error becomes significant, ABS and stability control calibration is affected, and on some vehicles transmission shift points are calculated from wheel speed." It is the only tire maker in our set to name a percentage. Michelin says all-wheel-drive and four-wheel-drive systems typically require all four tires to have the same circumference, and Toyo warns that for 4WD and AWD vehicles even small differences in outside diameter may cause drivetrain damage, but neither gives a percentage. Here is our arithmetic, starting from the example size:
Green means inside BFGoodrich's "about 3 percent" diameter guide and amber means just past it (3.21 percent; BFGoodrich gives no cutoff, so this is our reading); neither colour says anything about load index, rim width, clearance or your car maker's approval. The last column assumes the speedometer is set for the original size and ignores its tolerance. A larger overall diameter makes the car travel farther per wheel turn, so the speedometer reads low and the car is faster than shown: at 60 mph indicated, the 225/65R17 in the table is going about 61.9 mph. A smaller diameter does the opposite, and the odometer drifts the same way.
Three things the table cannot show. First, the load index still applies. BFGoodrich: "Your new tire's load index must meet or exceed the original specification. Load index requirements do not drop just because the tire is a different size." Second, the wheel must be right for the tire. Toyo, in its list for light-truck replacement tires: "Tires should be mounted on approved rim widths." FMVSS 110 requires the rims a car is built with to match a rim the tire maker lists as suitable; a wheel you add later is your check to make. Third, a wrong-size wheel reaches beyond the tire. Mazda's 2026 Mazda3 manual lists what it may adversely affect: tire fit, wheel and bearing life, ground clearance, snow-chain clearance, speedometer calibration, headlight aim, bumper height and the tire pressure monitoring system.
None of this says plus sizing is wrong. It says the choice is a package. If you want one, ask a tire professional to confirm each item for your car, and check your manual for any advice against it, before you buy.
Mixing tires and replacing fewer than four
Most buyers replace tires in pairs or one at a time, so the mixing rules matter more than they sound. The sources agree more than they disagree, and the differences are worth knowing.
Where the sources agree. If you replace only two, the US Tire Manufacturers Association, the Tire Industry Association, Michelin, Hankook, Toyo, GM and Nissan put the new tires on the rear axle; Subaru, Mazda, Hyundai and Honda name no axle. The reason, in the Tire Industry Association's words: "If the new tires are installed on the front axle with worn tires on the rear, then the vehicle may be more likely to lose control when turning in wet or slippery conditions." Michelin: "If replacing only two, put the new tires on the rear axle regardless of which are more worn." All four tires should have the same size, and on all-wheel-drive and four-wheel-drive cars, diameter differences are the worry: Toyo: "For 4WD and AWD vehicles, even small differences in outside diameter may cause drive-train damage or mechanical malfunction." Toyo also says that in some cases the vehicle manufacturer may specifically advise against replacing less than all four tires.
Where they differ. Subaru's North American manual (edited December 2019) is the strictest in our set for all-wheel-drive models: all four tires must be the same in size, speed symbol, load index, circumference, construction, manufacturer, brand and tread pattern, and degrees of wear, or serious mechanical damage could be caused to the drivetrain. Mazda's 2026 Mazda3 manual says replacing just one tire is dangerous and strongly recommends all four. Hyundai recommends replacing the front or rear pair together, and Honda says it is best to replace all four, or failing that the front or rear in pairs. The Tire Industry Association notes that on certain all-wheel-drive and four-wheel-drive vehicles the makers require all four, so your own manual decides.
Speed symbols and the axle. Michelin says combining different speed ratings is not recommended and limits the vehicle to the lowest-rated tire's speed. If it happens, Toyo (with the exception of winter or snow tires) and BFGoodrich say the lower-rated tires go on the front axle. If the new pair is also the lower-rated one, that rule and the rear-axle rule pull in opposite directions; ask the shop and your manual which one wins for your car.
Winter sets. Nitto: "Winter (snow) tires should always be fitted in matched sets of four." Michelin says two winter tires are not recommended and a full set of four should always be fitted. Honda says to mount winter tires of the original size and load range on all four wheels. Toyo says it is always preferable to apply them to all wheel positions and that they must not go on the front axle only. Nitto, Michelin, Toyo and Honda say four; only the Tire Industry Association says that if just two winter tires are installed, they go on the rear.
The spare. The makers' speed limits for a temporary spare differ. Michelin and Hyundai's 2023 Santa Fe manual (for its compact spare) say 50 mph. GM's manual says its temporary-use full-size spare should not be driven over 112 km/h (70 mph), or 88 km/h (55 mph) with a trailer. Subaru says to use the original temporary spare stored in the car, because other sizes may cause severe mechanical damage to the drivetrain.
If a car pulls to one side or shakes at speed after a tire change, that is a diagnosis and not a sizing question; the guides on a car that pulls to one side, a car that shakes at highway speed and wheel alignment cost take it from there.
The traps: three ways a careful buyer still gets it wrong
Trap one: reading the sidewall maximum as the target. The big numbers molded into the sidewall are the most visible figures on the tire, and they are the wrong ones for filling it. Michelin says MAX PRESS is not the recommended operating pressure, and Hankook says do not apply tire pressure above it. Fill to the cold pressure on your car's tire placard, never to the maximum printed on the tire's sidewall.
Trap two: matching three numbers and ignoring the service description. A shopping page asks for width, aspect ratio and wheel diameter. It does not ask whether the tire is an LT or a P, whether it is XL, or whether the load index is at least the original. Michelin says that if the factory tires are XL, replacements must be XL, and that P-metric tires of the same size on a truck or SUV that came with LT tires leave you short on load capacity. A matching size is where the check starts; the service description, 98H in the example, is where it ends.
Trap three: letting the look decide the wheel. A larger wheel with a thinner tire is often chosen for appearance. The sources say the choice is a package: overall diameter within about 3 percent (BFGoodrich's guide), a load index that still meets the original, a rim the tire maker lists, and, as individual car makers warn, a size that does not upset ABS and stability control (Honda), the speedometer, headlight aim and the tire pressure warning (Mazda, Nissan). Check with the owner's manual and a tire professional first.
Three composite situations, decoded
These are composite scenarios, not individual customers. They are built only from the facts above, and the sizes in them are ours.
The online order. An owner reads the size from the tires on a crossover, 235/60R18, and orders it on a retailer's page, which offers one listing with a load index of 102 and one with 99. The placard says 235/60R18 102H. A load index of 99 (1,709 pounds a tire, on BFGoodrich's list) is lower than the 102 on the placard, and Michelin, Toyo, BFGoodrich and both trade groups say the index must not go below the original. The owner takes the 102 listing, checks it is not an XL or LT variant that differs from the placard, and asks the shop to show the DOT date before mounting.
The bigger wheels. A driver finds 19-inch wheels second-hand for a car sold on 225/60R17 tires, the example above. By our arithmetic, 245/45R19 is within 0.19 percent of the example's overall diameter, so the speedometer would barely change and BFGoodrich's 3 percent guide is met. But the load index must still meet the original, the rim width must be one the tire maker lists, and the car maker's manual may advise against any other size. The driver asks a tire shop and reads the manual's list of what a wrong size can affect.
The two tires. An owner of an all-wheel-drive wagon has two worn tires and a budget for two. The shop says to put the new pair on the rear, as the Tire Industry Association and Michelin also say. The owner's manual in this scenario says, as Subaru's does, that all four tires must be the same, including the degrees of wear. The owner asks the car maker's dealer whether a pair is acceptable. The manual decides; the shop's habit does not override it.
Where a free sound check fits, and where it does not
This is our side of the page, and it concerns PulsCar's free engine sound check. PulsCar's free sound check asks you to film your engine running for 30 seconds and returns a report by email in about ten minutes. The home page states the terms plainly: "No payment, no card, no subscription." If a clip cannot support a confident verdict, the report says "inconclusive", and you can record again.
It does not fit the questions on this page. A tire size is a number on rubber, and a recording of an engine cannot read it, check a load index or tell whether a wheel and tire will clear. It listens to an engine. If, after a tire change, the engine itself sounds different, a clip is a free extra. A sound or a vibration that seems to come from the tires or wheels means inspect first, as the next paragraph says.
A noise from the brakes, the steering or a wheel, a vibration through the steering wheel, or any red or flashing warning light means inspect first; do not record. A flashing check engine light is a different problem: pull over safely, switch the engine off and have it towed.
Film 30 seconds of the engine running on your phone. PulsCar returns a free report by email, or says it is inconclusive. It cannot read a tire. A noise from the brakes, the steering or a wheel, a vibration through the steering wheel, or any red or flashing warning light means inspect first; do not record. Free, no card, no subscription.
What this page cannot tell you
The limits of this page are as real as its sources, and a reader should know where the evidence stops.
- No NHTSA tire statements. NHTSA's tire pages blocked our access on 5 November 2026. The federal text on this page comes from the regulations on the eCFR, and we have no NHTSA statement on tire age.
- Trade groups. The Tire and Rim Association and Rubber Manufacturers Association sites did not connect, and the Tire Industry Association's load index and speed rating charts are not in its page text.
- No full load index table. We print 91 to 100 and one rule for everything else. The other ranges of Hankook's table did not agree with other makers' figures, so we did not use them. For speed symbol Q, Toyo and General Tire print 99 mph and Hankook's table prints 94 mph; we show the first two.
- Tire makers. Goodyear's US pages did not load as text, so we used its European site. Continental's sidewall guide was blocked, Cooper's pages returned an error, Falken's did not connect and Kumho's carried no sidewall guide.
- Car makers. We read specific manuals: GM (2019 Tahoe and Suburban), Hyundai (2023 Santa Fe), Kia (web pages), Nissan (2019 Titan), Subaru (a North American manual edited in 2019), Honda (a 2017 Civic Type R tire sheet), Mazda (Mazda3 and CX-5 pages) and a Toyota maintenance-data page. Ford's pages, Toyota's manual PDFs and Honda's manual site were blocked or unavailable. We have no pages from Volkswagen, Jeep, Ram, Tesla, Volvo or BMW. Your own manual is the one that counts.
- No federal rule we could cite for the speed symbol or load index on the sidewall. FMVSS 139 lists the maximum load rating (S5.5(d)) and FMVSS 110 leaves the load index and speed rating as an option on the placard (S4.3(i)). That is a finding about those texts, not a legal opinion.
- No testing, United States only. We did not test any tire or measure any speedometer; the arithmetic in the tables is ours.
Your action plan: before you shop, when you order, at the shop, and after
Before you shop
- Read your car's placard on the driver's door pillar and photograph it: the size, the pressures, and any load index and speed rating. Read the tire page of the owner's manual for what the car maker says about changes.
- Compare the placard with the size on the tires on your car now. If they differ, find out why before you order.
- Read the DOT date on the tires you have, including the spare, as week then year.
When you order
- Match the size, then the load index and speed symbol, and the type letters (P, LT or none; XL if the original was XL). Never fit a tire whose load index is lower than the one your car was built with.
- Check any size that differs from the placard with a tire professional and with your owner's manual before you buy, and confirm the rim width, the overall diameter and the load index.
At the shop
- Ask to see the DOT date on the new tires before they go on.
- If you are replacing fewer than four, ask where the new tires go and check that against your manual. On an all-wheel-drive car, read the tire section of your manual before you order: some makers require all four tires to match, wear included (Subaru's 2019 manual does).
- Fill to the cold pressure on your car's tire placard, never to the maximum printed on the tire's sidewall.
- Ask whether the tire pressure monitoring system needs attention after the change; our tire pressure light guide explains what to expect.
After
- Keep the receipt with the sizes and DOT dates. If the car now pulls, shakes or sounds different, see the guides on pulling to one side, shaking at highway speed and wheel alignment cost.
Sources and documents
The facts on this page come from the pages below, read on 5 November 2026 and saved as text; advice marked as ours is ours, and so is the arithmetic.
- Federal regulations (eCFR, version of 1 November 2026): 49 CFR 574.5, tire identification; 49 CFR 575.104, uniform tire quality grading; 49 CFR 571.139, FMVSS 139; 49 CFR 571.110, FMVSS 110; 49 CFR 567.4, certification label
- Trade groups: US Tire Manufacturers Association, replacing tires, tire facts and tire care essentials; Tire Industry Association, reading a tire sidewall and tire replacement
- Tire makers: Michelin, tire markings explained, load rating and speed rating, mixing tire brands, tire safety tips and driving on a spare; BFGoodrich, reading a tire sidewall, changing tire size, the right tire for your vehicle and speed rating; Bridgestone, tire specs; Toyo, reading a sidewall, FAQ, plus sizing, tire safety and load and inflation tables; Hankook, load and speed, tire pressure and replacement; Pirelli, tire markings; Yokohama, sizing, DOT marking, UTQG and markings and warnings; General Tire, what the numbers mean, how to read your sidewall and the DOT number; Nitto, reading a sidewall and a technical bulletin on replacement tires; Firestone, tire decision guide; Goodyear Europe, sidewall markings
- Car makers: GM, 2019 Chevrolet Tahoe and Suburban owner's manual; Honda, 2017 Civic Type R tire information sheet; Hyundai, 2023 Santa Fe owner's manual; Kia, owner's manual pages on tire labeling and tire terminology; Mazda, Mazda3 2026, Mazda3 2020, CX-5 2021 and CX-5 2018; Nissan, 2019 Titan owner's manual; Subaru, North American owner's manual; Toyota, an owner's manual maintenance-data page
- PulsCar: home page and free sound check
- Attempted, access blocked or not available, not quoted: NHTSA tire pages (HTTP 403), Tire Rack and the Department of Transportation site (HTTP 403), Consumer Reports (HTTP 404), the Tire and Rim Association and the Rubber Manufacturers Association (no connection), Ford and Toyota manual PDFs (HTTP 403), Honda's manual site (HTTP 503), Continental's sidewall guide (HTTP 403)
Prices verified 5 November 2026: this page carries no prices; the only figures are specifications printed in the sources and our own arithmetic. Technical documents verified 5 November 2026: federal regulations, trade group pages, tire maker pages and car maker manuals. Written by PulsCar, which makes an app for a car's diagnostic port and the free sound check.
