Saturn 2009 ASTRA - Manual (Page 228)

Tire Size
The following illustration shows
an example of a typical
passenger 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
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 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.
Tire Terminology and
Definitions
Air Pressure : The amount of
air inside the tire pressing
outward on each square inch
of the tire. Air pressure is
expressed in pounds per square
inch (psi) or kilopascal (kPa).
Accessory Weight : This means
the combined weight of optional
accessories. Some examples
of optional accessories are,
automatic transmission, power
steering, power brakes, power
windows, power seats, and air
conditioning.
Aspect Ratio : The relationship
of a tire’s height to its width.
Belt : A rubber coated layer of
cords that is located between the
plies and the tread. Cords may
be made from steel or other
reinforcing materials.
9-44 Vehicle Service and Care