280 Vehicle Care
(6) Uniform Tire Quality Grading (UTQG) :
Tire manufacturers are required to grade
tires based on three performance factors:
treadwear, traction, and temperature
resistance. For more information, see
Uniform Tire Quality Grading 0
292.
(7) Maximum Cold Inflation Load Limit
:
Maximum load that can be carried and the
maximum pressure needed to support that
load.
Tire Designations
Tire Size
The example shows a typical passenger
vehicle tire size.
Passenger (P-Metric) Tire
(1) 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.
(2) Tire Width
: The 3-digit number
indicates the tire section width in
millimeters from sidewall to sidewall.
(3) Aspect Ratio
: A 2-digit number that
indicates the tire height-to-width
measurements. For example, if the tire size
aspect ratio is 75, as shown in item (3) of
the illustration, it would mean that the
tire's sidewall is 75 percent as high as it is
wide.
(4) 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.
(5) Rim Diameter
: Diameter of the wheel
in inches.
(6) Service Description
: These characters
represent the load index and speed rating
of the tire. The load index represents the
load carrying 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 kPa
(kilopascal) or psi (pounds per square inch).
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 kPa
(kilopascal) or psi (pounds per square inch)
before a tire has built up heat from driving.
See Tire Pressure 0
282.
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