Tire Size Explained: How to Read Numbers and Letters

Size string at a glance

A tire sidewall contains a standardized size string. The letters and numbers are read from left to right, and each group has a specific meaning. Federal Motor Vehicle Safety Standards define many of the terms behind those markings.

The example P215/65R15 95H is a common format, though your vehicle may use LT, ST, or another prefix. Before buying, get the exact size from the vehicle placard and the owner's manual. If the size on the sidewall and the placard do not match, start with the placard.

Vehicle type letters: P, LT, ST and T

The letters before the main size identify the intended vehicle class. A passenger car tire is defined as a tire intended for passenger cars, multipurpose passenger vehicles, and trucks with a gross vehicle weight rating of 10,000 pounds or less. A size that begins with P matches that passenger car category.

A light truck (LT) tire is designated by its manufacturer as primarily intended for use on lightweight trucks or multipurpose passenger vehicles. If your truck or van calls for an LT size, compare tires within the light truck tire group.

Separate federal rules cover specialty tires such as ST trailer tires and FI farm tires, along with motorcycle tires. A T-type spare tire is also covered by the older passenger car standard and is intended as a temporary spare.

Section width and aspect ratio

The number before the slash is the section width code. FMVSS defines section width as the linear distance between the exteriors of the sidewalls of an inflated tire, excluding sidewall labels, decoration, and protective bands. See tire section width for the full breakdown.

The two-digit number after the slash is the aspect ratio, the sidewall height as a percentage of section width. A lower aspect ratio generally means a shorter sidewall and a wider look; a higher aspect ratio generally means a taller sidewall and more sidewall cushioning. For more detail, read tire aspect ratio.

Construction letter: R and B

The letter after the aspect ratio tells you how the plies are arranged. A radial ply tire has ply cords that extend to the beads at substantially 90 degrees to the centerline of the tread. Most modern tire sizes use R for radial construction.

A bias ply tire has ply cords laid at alternate angles substantially less than 90 degrees to the tread centerline. Because the internal structure affects ride, flex, and rim fit, keep the same construction family when you replace tires. See tire construction types and tire rim width range.

Rim diameter and overall dimensions

The last number in the main size is the rim diameter code. FMVSS defines rim diameter as the nominal diameter of the bead seat and rim width as the nominal distance between rim flanges. The rim must be one that the tire manufacturer lists as suitable for the tire.

The inflated tire also has an outer diameter and an overall width. Overall width includes sidewall elevations such as labels and protective ribs, while section width does not. This distinction matters when you compare tires for fitment. See tire dimensions diameter before changing sizes.

Load index, speed rating and extra load

The group at the end of the size is the service description. It typically begins with a load index and ends with a speed rating. Federal standards define load rating as the maximum load a tire is rated to carry for a given inflation pressure.

The load index converts to a load rating when the tire is inflated to the corresponding pressure; the speed rating letter states the certified speed capability of that service description. Keep the load index at or above the original equipment value. For tables and details, see tire load index rating and tire speed rating guide.

Extra load and reinforced tires are designed to operate at higher loads and higher inflation pressures than the corresponding standard tire. An XL marking or the word reinforced is a load-capability signal, not a license to ignore the vehicle placard. Read more in extra load tires XL.

Snow tires and UTQG grades

Other sidewall markings are not part of the size string but still help you read a tire. The Uniform Tire Quality Grading standards put treadwear, traction, and temperature resistance grades on passenger tires. Traction grades use AA, A, B, or C, and temperature resistance grades use A, B, or C.

The federal snow tire definition requires a traction index of at least 112 on a standard snow test and an Alpine Symbol on at least one sidewall. That alpine marking is the quick way to confirm a true winter tire. Read the snow tire 3PMSF symbol for more.

Before you buy

Always start with the tire size on the vehicle placard, usually in the driver side door jamb or in the owner's manual. Federal tire selection rules are designed to prevent overloading: the vehicle maximum load on the tire must not be greater than the tire's maximum load rating.

Inflate to the vehicle manufacturer's recommended cold inflation pressure, not the maximum pressure printed on the sidewall. The proper pressure is usually listed on the driver side door jamb or glove box and in the owner's manual. Keeping tires properly inflated helps fuel economy; under-inflated tires can lower gas mileage. See tire pressure basics.

When buying replacements, match the size, load rating, and construction to the original equipment. If you are considering a different size, read the tire sizes interchangeability guidance before you commit.

What to pick for your use

Questions

What does the P in a tire size mean?

P identifies the tire as a passenger car tire under FMVSS definitions: a tire intended for passenger cars, multipurpose passenger vehicles, and trucks with a gross vehicle weight rating of 10,000 pounds or less.

What does the R in a tire size mean?

R stands for radial construction. A radial ply tire has ply cords extending to the beads at substantially 90 degrees to the centerline of the tread. Bias ply tires use cords at alternate angles substantially less than 90 degrees.

What do the numbers after the slash in a tire size mean?

The first number is the section width, measured between the exteriors of the sidewalls of an inflated tire. The second number is the aspect ratio, which describes sidewall height as a percentage of section width.

What is the service description at the end of a tire size?

The service description combines a load index and a speed rating. Federal standards define load rating as the maximum load a tire is rated to carry for a given inflation pressure. The speed rating is the letter code for the tire's certified speed capability.

Can I use a tire size that is not on my vehicle placard?

Start with the placard size and do not exceed the vehicle load requirements. If you want to change size, study the plus sizing guide and interchangeability guidance, and keep the rim diameter and load rating compatible.

What is the Alpine Symbol on winter tires?

FMVSS defines a snow tire as one that attains a traction index of at least 112 in a standard snow test and is marked with an Alpine Symbol on at least one sidewall. It is the accepted confirmation of a winter tire rather than a seasonal label.