What Do the Numbers on a Tire Mean?

Tires hold a lot of information if you know how to read the numbers and letters on them, but many people don’t know how to read a tire sidewall for basic data like tire size. So, what do the numbers on a tire mean? The good news is that the answers are relatively straightforward, but the bad news is that it’s a great deal of info to memorize. Even professional mechanics may need to refer to load index, speed rating, and tire size charts to verify exactly what tire numbers mean. Read over the detailed explanation below if you need a clear method for reading a tire sidewall.

Tire Sizes Explained

When you need new tires to replace worn ones, it’s important to know the size of your old tires—especially if you’re not buying a full set. Tire size can be found on the sidewall, represented by an alphanumeric code indicating the tire type, width, aspect ratio, construction type, wheel diameter, load index, and speed rating.

Tire Type

Check the tire’s sidewall for a series of letters and numbers 11 to 13 characters in length. The first character should be a letter indicating the tire type, referring to the type of vehicle for which the tire is designed.

P: If the code starts with a P, then the tire is made for passenger vehicles like sedans, crossovers, minivans, and most SUVs and pickup trucks. These are commonly known as P-metric tires.

LT: Full-size pickup trucks and SUVs may have tires with the LT designation, which stands for “light truck.” These tires are typically made for carrying heavy loads or towing trailers.

ST: Typically seen on a variety of trailers, ST stands for “special trailer.” Tires with the ST designation should never be used on cars, vans, SUVs, or any other type of passenger vehicle.

No Letter: Some tire size codes don’t begin with a letter, so they fall into a separate category. These are typically European metric sizes. While those tires are still measured in millimeters and could be similar to a P-metric tire size, they may have a different load capacity.

Tire Width

There should be a three-digit number after the initial letter(s). This number indicates the width of the tire in millimeters. So, if the number listed is 215, then the tire width measures 215 mm. This measurement is vital for finding replacement tires, but the tire’s width is also needed to calculate the aspect ratio.

Aspect Ratio

The next symbol on the tire sidewall is a forward slash, followed by two digits representing the aspect ratio, or the ratio of tire height to tire width. Tire height is measured from the wheel rim to the top of the tire tread, but it’s written as a percentage on the sidewall. For this reason, tire width is required to calculate tire height.

If the first three digits representing the tire width are 215 and the following aspect ratio digits are 65, the tire is 215 mm wide and the height is 65 percent of the width. You can calculate the exact measurement relatively easily with the following formula, where AR equals the aspect ratio, TW equals tire width, and TH equals tire height.

(AR/100) x TW = TH
(65/100) x 215 mm = 139.75 mm

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Construction Type

One or more letters, an R or D, should follow the two tire measurements. On some tires, there may also be an F, but that should still be followed by an R or a D. This letter indicates how the tire is constructed.

R: This designation is the most common for modern tires. The R stands for “radial tires,” which have superior road grip, gas mileage, and ride comfort. Radial tires are made with multiple layers of rubber-coated cords laid perpendicular to the direction of travel. These cords are made using a blend of polyester, steel, and fabric to improve overall tire durability.

D: The D designation corresponds to bias tires. These tires have diagonal or crisscrossed cord plies and are sometimes used on motorcycles and trailers. However, this tire construction type isn’t common for the average passenger vehicle.

Wheel Diameter

Two digits should be listed after the construction type. These numbers express the diameter of the wheel in inches. This means that if the number provided is 17, then the tire is designed to fit on a 17-inch wheel. Mind that this number isn’t a measurement of the tire’s tread-to-tread diameter; it represents the internal diameter from the two bead seat areas on the inside of the tire so that you know the appropriate wheel size for the tire.

Load Index

The tire load index is a code that references the amount of weight a single tire can handle. It’s listed as a two- or three-digit number after the wheel diameter. To determine the weight in pounds, refer to a load index chart like the one provided below. Once you find the load capacity for one tire, then (assuming all of your tires’ load capacities match) you can calculate your vehicle’s maximum load by multiplying the single tire load capacity by four.

75 = 852 lbs
78 = 937 lbs
81 = 1,019 lbs
84 = 1,102 lbs
87 = 1,201 lbs
90 = 1,323 lbs
93 = 1,433 lbs
96 = 1,565 lbs
99 = 1,709 lbs
102 = 1,874 lbs
105 = 2,039 lbs
108 = 2,205 lbs
111 = 2,403 lbs
114 = 2,601 lbs
117 = 2,833 lbs
120 = 3,086 lbs

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Speed Rating

Similar to the load index, tire speed ratings indicate the maximum speed for which a tire is rated. Tire speed rating is often represented by a letter, but it can also be a letter and a number. In rare cases where the speed rating exceeds 186 mph, it may be designated by a ZR followed by (Y). See the speed rating chart below for a complete list of speed ratings and their meanings.

A1 = 3 mph
A2 = 6 mph
A3 = 9 mph
A4 = 12 mph
A5 = 16 mph
A6 = 19 mph
A7 = 22 mph
A8 = 25 mph
B = 31 mph
C = 37 mph
D = 40 mph
E = 43 mph
F = 50 mph
G = 56 mph
J = 62 mph
K = 68 mph
L = 75 mph
M = 81 mph
N = 87 mph
P = 93 mph
Q = 99 mph
R = 106 mph
S = 112 mph
T = 118 mph
U = 124 mph
H = 130 mph
V = 149 mph
W = 168 mph
Y = 186 mph
(Y) = >186 mph
ZR = This may appear on tires rated above 149 mph. It’s also used to indicate >186 mph when accompanied by the (Y) symbol.

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U.S. Department of Transportation Number

Every tire sold in the U.S. needs to have a Department of Transportation (DOT) identification number. This number indicates a tire has passed minimum safety requirements for sale in the U.S., and it also includes manufacturer-specific coding to denote what company manufactured the tire, where it was made, and digits for tracking the sale of the tire in case it must be recalled.

The last four digits of this DOT serial number are the most useful for the average driver. The first two digits represent the week the tire was made, while the last two digits represent the year. If this number is 2620, then the tire was manufactured in the 26th week of 2020.

Uniform Tire Quality Grade

Manufacturers selling tires in the U.S. are also required by the DOT to grade their tires according to Uniform Tire Quality Grade (UTQG) standards to rate treadwear, traction, and temperature resistance.

Treadwear

The treadwear rating is the first UTQG figure provided, and it’s generated using a 7,200-mile wear test. Tires are graded based on the rate of wear they would endure after being driven for 7,200 miles. The ratings are relatively straightforward: A tire with a grade of 100 will wear out three times faster than a tire with a grade of 300. Similarly, a tire with a grade of 600 will last twice as long as a tire with a grade of 300.

Traction

Traction ratings are based on tire grip and a tire’s ability to stop in a straight line on wet concrete or asphalt. Traction ratings include AA, A, B, or C. Just like in academic grading, higher letter grades mean better ratings, with AA being the best tire traction rating.

Temperature

Tire temperature ratings include A, B, and C, and tires with an A rating are able to withstand greater temperatures and dissipate heat more quickly than lower-rated tires. Depending on a tire’s design, it will have a certain level of resistance to heat; higher temperature ratings translate to better heat resistance at higher speeds.

M+S

The letters M+S on a tire sidewall stand for “mud and snow.” Expect to see this code on all-weather tires designed for muddy conditions and light snow. The M+S code can also be followed by an E (M+SE) for studded snow tires. If you regularly drive through heavy snowfall, you can additionally look for tires featuring a mountain and snowflake on the sidewall—this symbol means it’s a winter tire.

Rotation Arrows

Some tires may be listed as directional or unidirectional. This means the tire needs to be installed facing in a specific direction. To make it simple, the correct direction is represented by an arrow. This rotation arrow points in the direction which the tire should rotate when the vehicle is moving forward. Tires that aren’t unidirectional will not include a rotation arrow, so don’t be surprised if this symbol is missing from your sidewall.

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