GMC 2009 Sierra - Manual (Page 374)

(A) Light Truck (LT-Metric) Tire : The
United States version of a metric tire sizing
system. The letters LT as the first two characters
in the tire size means a light truck 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 light truck (LT-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.
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 Tire Pressure : The amount of air pressure
in a tire, measured in pounds per square inch
(psi) or kilopascals (kPa) before a tire has built up
heat from driving. See Inflation - Tire Pressure
on page 5-65.
5-62