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(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: The service description
indicates the load range and speed rating of a tire.
The load index can range from 1 to 279. Speed ratings
range from “A” to “Z”. The light truck tire size example
above shows dual or single tire configurations.
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.
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 Inflation 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. See Inflation - Tire Pressure on page 5-64.
Curb Weight: This means 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 motor vehicle
safety standards. The DOT code includes the Tire
Identification Number (TIN), an alphanumeric designator
which can also identify the tire manufacturer, production
plant, brand and date of production.
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