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(F) Tire Size : A combination of letters and
numbers define a tire’s width, height, aspect
ratio, construction type and service description.
The letter T as the first character in the tire
size means the tire is for temporary use only.
(G) TPC Spec (Tire Performance Criteria
Specification)
: Original equipment tires designed
to GM’s specific tire performance criteria have a
TPC specification code molded onto the sidewall.
GM’s TPC specifications meet or exceed all federal
safety guidelines.
Tire Size
The following illustration shows an example of a
typical passenger vehicle 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.
(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 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 index and speed rating of the
tire. The load index represents the load carry
capacity a tire is certified to carry. The speed
rating is the maximum speed a tire is certified
to carry a load.
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