Cadillac 2005 STS - Manual (Page 345)

Tire Size
The following illustration shows an example of a typical
passenger vehicle 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 60, as shown in item C of
the illustration, it would mean that the tire’s sidewall is
60 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: 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.
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