GMC 2008 Envoy - Manual (Page 354)

Tire Size
The following examples show the different parts of
a tire size.
(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.
(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
5-60