2004 GMC Envoy Owner's Manual - Page 368

(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 illustration, it would mean that the tire's sidewall is 75% 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". Light Truck (LT-Metric) Tire (A) Light Truck (LT-Metric) Tire: The United States version of a metric tire sizing system. The letter "LT" as the first two characters in the tire size means a light truck 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 "75," as shown in item "C" of the illustration, it would mean that the tire's sidewall is 75% as high as it is wide. 5-60