(G) TPC Spec (Tire Performanc e Criteria
Specification)
: Original equipment tires designed
to GM's specific tire performa nce 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 passenge r 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 Associatio n. A tire
size without the letter P as the first character is
certified to European standards.
(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‐w idth measurem ents.
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 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.
6-62
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