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Tire sizes express the tire’s various measurements in a standardized format written on the side of the tire, also called the sidewall. The three measurements included in all tire sizes are section width, aspect ratio, and diameter. Depending on their intended use, some tire sizes also include information such as speed rating.
Tire sizes are expressed in the format WWW/AAXDD. WWW is the tire’s section width, measured in millimeters. AA is the aspect ratio or profile of the tire, which expresses the tire’s height as a percentage of its width. X is a letter indicating the tire’s internal construction. DD is the diameter in inches of the wheel that the tire is intended to be mounted on. An example of a tire size written in this format is 225/50R16.
This particular tire has a section width of 225 millimeters (8.86 inches). Section width is measured from the widest point of the outer sidewall - the side with the sizes and brand name that faces out from the vehicle - to the widest point of the inner sidewall when the tire is mounted on a specified width wheel. Narrower tires have lower section width numbers.
The next number is the aspect ratio. The sizes separate the section width from the aspect ratio with a slash. This tire has an aspect ratio of 50, meaning that the profile, or sidewall height measured from wheel to tread, is 50% of the section width. This number is also called section height; the higher the number, the taller the sidewall.
Tire sizes include a letter following the section height that is not a measurement, but an identification of the tire’s internal construction. This “R” indicates radial construction, in which the tire’s body plies “radiate” outward from the wheel’s center. A “D” indicates plies that crisscross diagonally, used for light truck or spare tires. Sizes with a “B” are belted and nearly obsolete.
The final number indicates the tire and wheel diameter to be used together - in this case, 16 inches. These inch rim sizes are used on most passenger vehicles, while some tires used on heavy-duty trucks or trailers use “half inch” sizes, e.g. 14.5. Some manufacturers' sizes express rim diameter in millimeters, or include different dimensions for the inside and outside edges of the tire. These sizes are intended for use with specific vehicles and should not be combined with traditional tire sizes.
Tire sizes follow the rim diameter with a service description, a code indicating load capacity and speed rating. This service description is required on all tires manufactured since 1991, except for Z-speed rated tires, which express sizes with a “Z” following the aspect ratio.