GMC Cadillac CTS 2004 Owner's Manual page 356

Tire Size
The following illustration shows an example of a typical
passenger car tire size.
(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.
5-62
(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.
(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 speed rating of the tire. The load
index represents the load carry 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".