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 rst 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 range and speed rating
of the tire. The load index
represents the load carry
capacity a tire is certi ed to
carry. The load index can range
from 1 to 279. The speed
rating is the maximum speed
a tire is certi ed to carry
a load. Speed ratings range
fromAtoZ.
Vehicle Service and Care 9-45
Tire Terminology and
De nitions
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.
Bead
:The tire bead contains
steel wires wrapped by steel
cords that hold the tire onto
the rim.
Bias Ply Tire
:A pneumatic tire
in which the plies are laid at
alternate angles less than
90 degrees to the centerline of
the tread.
Cold Tire Pressure
:The
amount of air pressure in a tire,
measured in pounds per square
inch (psi) or kilopascals (kPa)
before a tire has built up heat
from driving. SeeInflation - Tire
Pressure on page 9-49.Curb Weight
:The weight of a
motor vehicle with standard
and optional equipment including
the maximum capacity of fuel,
oil, and coolant, but without
passengers and cargo.
DOT Markings
:A code molded
into the sidewall of a tire
signifying that the tire is in
compliance with the U.S.
Department of Transportation
(DOT) motor vehicle safety
standards. The DOT code
includes the Tire Identi cation
Number (TIN), an alphanumeric
designator which can also
identify the tire manufacturer,
production plant, brand, and date
of production.
GVWR
:Gross Vehicle Weight
Rating. SeeLoading the Vehicle
on page 8-30.
9-46 Vehicle Service and Care
Wheel Alignment and
Tire Balance
The tires and wheels on your vehicle
were aligned and balanced carefully
at the factory to give you the longest
tire life and best overall performance.
Adjustments to wheel alignment and
tire balancing will not be necessary
on a regular basis. However, if you
notice unusual tire wear or your
vehicle pulling to one side or the
other, the alignment might need to be
checked. If you notice your vehicle
vibrating when driving on a smooth
road, the tires and wheels might
need to be rebalanced. See your
dealer/retailer for proper diagnosis.
Wheel Replacement
Replace any wheel that is bent,
cracked or badly rusted or corroded.
If wheel nuts keep coming loose, the
wheel, wheel bolts and wheel nuts
should be replaced. If the wheel
leaks air, replace it (except some
aluminum wheels, which can
sometimes be repaired). See your
dealer/retailer if any of these
conditions exist.
Your dealer/retailer will know the
kind of wheel you need.
Each new wheel should have
the same load-carrying capacity,
diameter, width, offset and be
mounted the same way as the
one it replaces.
If you need to replace any of the
wheels, wheel bolts or wheel
nuts, replace them only with new
GM original equipment parts.
This way, you will be sure to have
the right wheel, wheel bolts and
wheel nuts for the vehicle.{CAUTION
Using the wrong replacement
wheels, wheel bolts, or wheel nuts
on your vehicle can be dangerous.
It could affect the braking and
handling of your vehicle, make
your tires lose air and make you
lose control. You could have a
collision in which you or others
could be injured. Always use the
correct wheel, wheel bolts, and
wheel nuts for replacement.
Notice:The wrong wheel can
also cause problems with bearing
life, brake cooling, speedometer
or odometer calibration, headlamp
aim, bumper height, vehicle
ground clearance, and tire or tire
chain clearance to the body and
chassis.
SeeChanging a Flat Tire on
page 9-75for more information.
9-64 Vehicle Service and Care
