Tire terminology and definitions – Cadillac 2004 XLR User Manual

Page 277

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(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.

(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 the speed rating of a tire. The
load range represents the load carrying 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”.

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.

Belt: A rubber coated layer of cords that is located
between the plies and the tread. Cords may be made
from steel or other reinforcing materials.

Bead: The tire bead contains steel wires wrapped by
steel cords that hold the tire onto the rim.

Bias Ply Tire: A pneumatic tire in which the plies are
laid at alternate angles less than 90 degrees to the
centerline of the tread.

Cold Inflation Pressure: The amount of air pressure in
a tire, measured in pounds per square inch (psi)
before a tire has built up heat from driving. See
Inflation - Tire Pressure on page 5-57.

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