Every tire is marked with an alphanumerical code on its sidewall that contains many of the tire's key characteristics. A tire sidewall code consists of two sections: the tire size, which includes the tire's dimensions and construction; and the service description, which includes the load index and speed rating. This page will explain how to read each part of a tire's sidewall code.
A tire size may start with a "P", "LT", "ST", or no letters at all. These letters refer to one of several tire categories, based on the tire's intended purpose and sidewall construction:
The first number in a tire size (before the slash) is the Section Width. This indicates the tire's cross-section width, measured as the distance from the widest point of the tire's outer sidewall to the widest point of its inner sidewall. A tire's section width is given in millimeters.
The second number in a tire size (after the slash) is the Sidewall Aspect Ratio, also referred to as the tire's profile. This is the ratio between the tire's sidewall height and its section width. A higher aspect ratio indicates a higher sidewall, while a lower aspect ratio indicates a lower sidewall. A higher sidewall profiles tends to provide greater ride comfort, while a lower sidewall profile offers more responsive handling.
The letter after the aspect ratio refers to the tire's internal construction. Passenger and light truck tires have an "R" to indicate radial ply construction, where the ply cords run up the sidewalls and directly across the tire from side to side, "radiating" from the tire's center. Very old tires may feature a "D" to indicate bias ply or diagonal construction, where the ply cords crisscross diagonally across the tire. Even older tires may have a "B" to indicate bias-belted construction, where the ply cords run diagonally over the tire and are reinforced with steel belts. The vast majority of tires today use radial ply construction.
Some high-performance tires may include a "Z" before the "R" to indicate that the tire is rated for sustaining speeds greater than 149 mph. This dates back to older tire standards that placed the speed rating symbol next to the construction symbol. Aside from the Z speed rating, today's tires indicate the speed rating in their service description instead.
The third number in a tire size (after the construction symbol) refers to the inside diameter of the tire for the purpose of wheel fitment. This number indicates which size of wheel the tire is compatible with, based on the wheel's diameter from bead seat to bead seat, measured in inches. A tire is designed solely to fit on a wheel that has a matching rim diameter, so a 16-inch tire may only be safely mounted on a 16-inch wheel.
The last part of a tire sidewall code is the service description. The number in the service description is the tire's Load Index, which refers to the tire's relative load-carrying capability and durability. Each load index value corresponds to a certain weight threshold, indicating the maximum load capacity that the tire is designed to carry. Most passenger tires have a load index ranging from 70 (max load of 739 lbs) to 110 (max load of 2,337 lbs).
The letter after the load index in a service description refers to the tire's speed rating, or its high-speed capability and heat dissipation. Each speed rating corresponds to a particular speed threshold, indicating the highest speed that the tire can sustain without heat failure. Note that a tire does not necessarily perform effectively at its maximum speed. Most passenger and truck tires today have a speed rating of S (max speed of 112 mph), T (max speed of 118 mph), H (max speed of 130 mph), V (max speed of 149 mph), or W (max speed of 168 mph). Trailer tires generally have a maximum speed of 65 mph unless their speed rating indicates otherwise.