Cadillac 2010 CTS Sport Sedan Owner's Manual page 362

The
temporary use tire or compact
spare tire should be inflated to
420 kPa (60 psi). For more
information on tire pressure and
inflation see
.
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.
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.
The following illustration shows
an example of a typical
passenger vehicle tire size.
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.
The three-digit
number indicates the tire section
width in millimeters from
sidewall to sidewall.
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.
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.