Tire SizeThe following illustration shows an example of a tire size.(A) P-Metric Tire: The United States version of ametric tire sizing system. The letter P as the firstcharacter in the tire size means a passenger vehicle tireengineered to standards set by the U. S. Tire andRim Association.(B) Tire Width: The three-digit number indicates thetire section width in millimeters from sidewall to sidewall.(C) Aspect Ratio: A two-digit number that indicatesthe tire height-to-width measurements. For example, ifthe tire size aspect ratio is 60, as shown in item C of theillustration, it would mean that the tire’s sidewall is60 percent as high as it is wide.(D) Construction Code: A letter code is used toindicate the type of ply construction in the tire. Theletter R means radial ply construction; the letter D meansdiagonal or bias ply construction; and the letter B meansbelted-bias ply construction.(E) Rim Diameter: Diameter of the wheel in inches.(F) Service Description: These characters representthe load range and speed rating of the tire. The loadindex represents the load carry capacity a tire is certifiedto carry. The load index can range from 1 to 279. Thespeed rating is the maximum speed a tire is certifiedto carry a load. Speed ratings range from A to Z.Tire Terminology and DefinitionsAir Pressure: The amount of air inside the tire pressingoutward on each square inch of the tire. Air pressureis expressed in pounds per square inch (psi) orkilopascal (kPa).Accessory Weight: This means the combined weightof optional accessories. Some examples of optionalaccessories are, automatic transmission, power steering,power brakes, power windows, power seats, and airconditioning.Aspect Ratio: The relationship of a tire’s height toits width.Belt: A rubber coated layer of cords that is locatedbetween the plies and the tread. Cords may be madefrom steel or other reinforcing materials.Bead: The tire bead contains steel wires wrapped bysteel cords that hold the tire onto the rim.5-53
