In the United States, tires sizes are commonly expressed in metric, often referred to as European measurements or standard measurements that may be referred to as American. Knowing how to decipher both types of measurements helps consumers purchase the proper tires.
European, or metric, tire sizing is expressed in a ratio that appears in a series of numbers, such as P225/50R16. The preceding letter indicates the type of vehicle that the tire is intended for. Common letters include P for passenger car, LT indicating light truck use and C for commercially-used tires. The following three-digit number is the tyre's section width, or essentially the width of the tire tread, expressed in millimetres. The two-digit number represents the tyre's sidewall height, also referred to as aspect ratio, as a percentage in relation to the section width. Letters, such as R ,indicate tire construction, in this case radial, and the final number indicates the diameter of the wheel, in inches, on which the tire should be mounted.
American tire sizes are expressed entirely in inches and appear in formats like 31/10.50R15. The first two-digit number indicates overall diameter of the tire, while the second number indicates the tread width, followed by the wheel size on which the tire should be mounted.
American, or standard tire sizes, are most commonly found on larger, off-road style tires used on pickups and sport utility vehicles.