Tire Size; Tire Terminology And Definitions - Cadillac SRX 2004 Owner's Manual

Tire Size

The following illustration shows the different parts of a
tire size code.
Passenger (P-Metric) Tire Example
(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 illustration, it would mean that the tire's sidewall is
75% as high as it is wide.
(D) Belt Rating 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". The light truck tire size example
above shows dual or single tire configurations.
Tire Terminology and Definitions
Air Pressure: The amount of air inside the tire pressing
outward on each square inch of the tire. Air pressure
is expressed in pounds per square inch (psi) or
kilopascal (kPa).
Accessory Weight: This means the combined weight
of optional accessories. Some examples of optional
accessories are, automatic transmission, power steering,
power brakes, power windows, power seats, and air
conditioning.
Aspect Ratio: The relationship of a tire's height to
its width.
5-65