Tires on passenger vehicles come in a variety of sizes, widths and heights. Using the tire size recommended by the vehicle manufacturer provides maximum performance and handling. A series of letters, numbers and symbols imprinted in the tire's sidewall allows you to determine the tire's dimensions and construction. U.S. and metric measurements are used in the code. For example, the recommended size for a two-wheel-drive 2002 Nissan Frontier is P225/70R15 100S.

Purpose

The letter that begins the series refers to the vehicle's intended purpose. In the example, "P" stands for "Passenger," indicating the tire is for passenger vehicles such as cars or light trucks. Others are "T" for temporary spares; "LT" for light trucks built for towing or heavier loads; and "ST" for "Special Trailer" tires, which go only on towed trailers and should never be used on passenger vehicles. Sometimes the code will end with "LT," which indicates vehicles with extra load and towing capacity. If it ends with "C," it is a European tire designed for commercial delivery trucks and vans.

Width

The number that follows the letter indicates the tire's width and is measured in millimeters. In the example, the tire's width is 225 mm. That number can be converted to inches by dividing by 25.4 (225/25.4 = 8.86 inches).

Profile

The next number represents the tire's height from rim to tread as a percentage of its width, called the "profile" or "side aspect ratio." In the example, the profile number is "70," meaning the tire's profile is 70 percent of its width. By multiplying the inch conversion of the width (8.86) by 70 percent (0.70), you can calculate the tire's profile in inches (8.86 x 0.70 = 6.2 inches).

Construction

The letter after the side aspect ratio indicates the internal construction of the tire. In the example, "R" shows that it is a radial tire, meaning the plies of cord that act as reinforcement on the interior of the tire "radiate" from an imaginary center of the wheel. Other letters used are "D," when the plies crisscross diagonally; and "B," which indicates crisscross plies that are reinforced with belts beneath the tread.

Rim Diameter

The next number expresses in inches the size of the wheel rim upon which the tire must be mounted. In the example, "15" shows the tire must be mounted on a 15-inch-wide rim.

Load Index and Speed Rating

The final numbers and letter show the tire's "load rating," or how much weight the tire can bear, and the "speed rating," which is the maximum speed the tire can run without falling apart. In the example, the load rating of "100" indicates a maximum load of 1,764 pounds, and the speed rating of "S" means the tire can handle a top speed of 112 mph.

Source:

TireRack.com: Tire Size Guide

TireRack.com: How to Read Speed Rating, Load Index & Service Descriptions

Cliffs Notes: Tire Sizes Explained

More Information:

Tire Size Calculator

Video: "How Tire Sizes Work"