2008 GMC Envoy Owner's Manual - Page 354

Tire Size The following examples show the different parts of a tire size. (C) Aspect Ratio: A two-digit number that indicates the tire height-to-width measurements. For example, if the tire size aspect ratio is 75, as shown in item C of the passenger (P-Metric) tire illustration, it would mean that the tire's sidewall is 75 percent as high as it is wide. (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: The service description indicates the load range and speed rating of a tire. The load index can range from 1 to 279. Speed ratings range from A to Z. Passenger (P-Metric) Tire (A) Passenger (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. 5-60