Chevrolet 2010 Impala Owner's Manual page 325

Tire Size
The following illustration shows an example of a
typical passenger vehicle tire size.
(A) Passenger (P-Metric) Tire
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.
The United States
:
(C) Aspect Ratio A two-digit number that
:
indicates the tire height-to-width measurements.
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
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
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.
A letter code is used
:
These characters
:
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