2004 Chevrolet Impala Owner's Manual - Page 324

Tire Size The following illustration shows an example of a typical passenger car tire size. (D) Construction Code: A letter code is used to indicate the type of ply construction in the tire. The letter "R" means radial ply construction; the letter "D" means diagonal or bias ply construction; and the letter "B" means belted-bias ply construction. (E) Rim Diameter: Diameter of the wheel in inches. (F) Service Description: These characters represent the load range and the speed rating of a tire. The load range represents the load carrying capacity a tire is certified to carry. The load index can range from 1 to 279. The speed rating is the maximum speed a tire is certified to carry a load. Speed ratings range from "A" to "Z". Tire Terminology and Definitions (A) P-Metric Tire: The United States version of a metric tire sizing system. The letter "P" as the first character in the tire size means a passenger vehicle tire engineered to standards set by the U. S. Tire and Rim Association. (B) Tire Width: The three-digit number indicates the tire section width in millimeters from sidewall to sidewall. (C) Aspect Ratio: A two-digit number that indicates the tire height-to-width measurements. For example, if the tire size aspect ratio is "70," as shown in item "C" of the illustration, it would mean that the tire's sidewall is 70% as high as it is wide. 5-62 Air Pressure: The amount of air inside the tire pressing outward on each square inch of the tire. Air pressure is expressed in pounds per square inch (psi) or kilopascal (kPa). Accessory Weight: This means the combined weight of optional accessories. Some examples of optional accessories are, automatic transmission, power steering, power brakes, power windows, power seats, and air conditioning. Aspect Ratio: The relationship of a tire's height to its width.