Tire Terminology and Definitions

Air Pressure: The amount of air inside the tire pressing outward on each square inch of tire. Air pressure is expressed in kPa (kilopascals) or psi (pounds per square inch).

Accessory Weight: The total weight of optional accessories. Some examples of optional accessories are automatic transmission, power windows, power seats, and air conditioning.

Aspect Ratio: The ratio of the tire's height to its width.

Belt: A rubber-coated layer of cords between the ply and the tread. The cords may be made of steel or other reinforcing materials.

Bead: The tire bead contains steel wires wrapped in steel cords that hold the tire on the rim.

Bias-Ply Tire: A pneumatic tire in which the plies are laid at alternating angles of less than 90 degrees to the centerline of the tread.

Cold Tire Inflation Pressure: The amount of air pressure in a tire, measured in kPa (kilopascals) or psi (pounds per square inch), before the tire heats up from driving. Refer to Tire Pressure.

Curb Weight: The weight of a vehicle with standard and optional equipment, including maximum capacities of fuel, oil, and coolant, but without passengers and cargo.

DOT Markings: The molded code on the tire sidewall indicates that the tire conforms to the United States Department of Transportation (DOT) Motor Vehicle Safety Standards. The DOT code includes the Tire Identification Number (TIN), an alphanumeric designation that can also identify the tire manufacturer, manufacturing plant, brand name, and date of manufacture.

Gross Vehicle Weight Rating (GVWR): The total weight rating of the vehicle. Refer to Vehicle Load Limits.

GAWR FR: Gross Axle Weight Rating of the front axle. Refer to Vehicle Load Limits.

GAWR RR: Gross Axle Weight Rating of the rear axle. Refer to Vehicle Load Limits.

Intended Outboard Sidewall: The side of an asymmetrical tire that must always face outward when the tire is mounted on the vehicle.

Kilopascal (kPa): A metric unit of air pressure.

Light Truck Tire (LT-Metric): A tire used on light trucks and some multipurpose passenger vehicles.

Load Index: An assigned number from 1 to 279 that corresponds to the load-carrying capacity of the tire.

Maximum Inflation Pressure: The maximum air pressure to which a cold tire may be inflated. The maximum air pressure is molded on the sidewall.

Maximum Load Rating: The load carrying capacity of the tire at the maximum permissible inflation pressure for that tire.

Maximum Loaded Vehicle Weight: The sum of curb weight, accessory weight, vehicle capacity weight, and production options weight.

Normal Occupant Weight: The number of designated seating positions multiplied by 68 kg (150 lbs). Refer to Vehicle Load Limits.

Occupant Distribution: Designated seating positions.

Outboard Sidewall: The side of an asymmetrical tire that has a specific side facing outward when the tire is mounted on the vehicle. The side of the tire that contains the whitewall, raised white letters, or raised manufacturer, brand, and/or model name molding is higher or deeper than the same molding on the other sidewall of the tire.

Passenger (P-Metric) Tire: A tire used on passenger cars and some light trucks and multipurpose vehicles.

Recommended Inflation Pressure: The tire pressure recommended by the vehicle manufacturer, as shown on the tire placard.

Refer to Tire Pressure on page 332 and Vehicle Load Limits.

Radial Tire: A pneumatic tire in which the ply cords extend to the beads and are laid at 90 degrees to the centerline of the tread.

Rim: The metal support for a tire, on which the tire beads seat.

Sidewall: The portion of a tire between the tread and the bead.

Speed Rating: An alphanumeric code assigned to a tire indicating the maximum speed at which the tire can carry a load.

Traction: The friction between the tire and the road surface. Provides grip.

Tread: The portion of a tire that comes into contact with the road.

Tread Wear Indicators: Narrow bands, sometimes called wear bars, that show across the tread of the tire when only 1.6 mm (1/16 inch) of tread depth remains. Refer to When to Replace Tires.

UTQGS (Uniform Tire Quality Grading Standards): A tire information system that provides consumers with ratings for tire traction, temperature, and treadwear. Ratings are determined by the tire manufacturer using government-specified test procedures. Ratings are molded into the tire sidewall. Refer to Uniform Tire Quality Grading.

Vehicle Capacity Weight: The number of designated seating positions multiplied by 68 kg (150 lbs), plus the rated cargo load. Refer to Vehicle Load Limits.

Maximum Vehicle Load on Tire: The load on an individual tire due to curb weight, accessory weight, occupant weight, and cargo weight.

Vehicle Placard: A label permanently attached to the vehicle showing vehicle capacity weight, original equipment tire size, and recommended inflation pressure. Refer to Tires and Loading. Placard under Vehicle Load Limits.