Chevrolet 2008 Express Owner's Manual page 306

Tire Size
The following examples show the different parts of
a tire size.
Passenger (P-Metric) Tire
Light Truck (LT-Metric) Tire
5-60
(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.
(A) Light Truck (LT-Metric) Tire
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.
The United States
:
The United
: