GMC 2010 Sierra Owner's Manual page 536

Light Truck (LT Metric) Tire
(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.
6-76
The United
:
(D) Construction Code
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) Load Range Load Range.
:
(G) Service Description
indicates the load index and speed rating of a
tire. If two numbers are given as in the example,
120/116, then this represents the load index for
single versus dual wheel usage (single/dual).
The speed rating is the maximum speed a tire is
certified to carry a load.
A letter code is used to
:
The service description
: