Tire Terminology and Definitions

Air Pressure

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

Accessory Weight

This is the total weight of optional accessories. Some examples of optional accessories are automatic transmission, power seats, and air conditioning.

Aspect Ratio

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

Belt

A layer of rubber-coated cord located between the plies and the tread. Cords may be made of steel or other reinforcing materials.

Bead

The tire bead consists of 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 an angle of less than 90 degrees to the centerline of the tread.

Cold Tire Pressure

The amount of air pressure in the tire, measured in pounds per square inch (psi) or kilopascals (kPa), before the tire heats up from driving.

Curb Weight

This is the weight of the vehicle with standard and optional equipment, including maximum fuel, oil, and coolant, but without passengers and cargo.

DOT Mark

The code molded into the tire sidewall indicates that the tire complies with the Federal Motor Vehicle Safety Standards of the U.S. Department of Transportation. The DOT code includes the Tire Identification Number (TIN), an alphanumeric designation that may also identify the tire manufacturer, manufacturing plant, brand, and date of manufacture.

Gross Vehicle Weight

Gross Vehicle Weight

GAWR FRT

Total load on the front axle.

GAWR RR

Total load on the rear axle.

Intended Outboard Sidewall

The side of an asymmetric tire that must always face outward when installed on the vehicle.

Kilopascal (kPa)

A metric unit of atmospheric pressure.

Light Truck (LT) Tire

A tire designated by the manufacturer as intended primarily for passenger cars, light trucks, or multipurpose passenger vehicles.

Load Ratings

The maximum load the tire is designed to carry at a given inflation pressure.

Load Index

An assigned number in the range 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 into the sidewall.

Maximum Load Rating

The load carrying capacity of the tire at the maximum permissible inflation pressure for this tire.

Maximum Loaded Vehicle Weight

The sum of the curb weight, payload, vehicle capacity weight, and production option weight of the vehicle.

Normal Occupant Weight

The number of passengers the vehicle is designed to carry, multiplied by 150 lb (68 kg).

Occupant Distribution

Designated seating positions.

Outboard Sidewall

An asymmetric tire has a specific side that must face outward when installed on the vehicle. The outward-facing sidewall carries white lettering or the manufacturer's brand name and/or model name molding, which is raised or deeper than the same molding on the inner sidewall.

Passenger (P-Metric) Tire

A tire used on passenger cars, some light trucks, and multipurpose vehicles.

Ply

A layer of parallel rubber-coated cords.

Pneumatic Tire

A mechanical device made of rubber, chemicals, fabric, steel, or other materials which, when mounted on a wheel, provides traction and contains gas or fluid that supports the load.

Tire Options Weight

The total weight of factory-installed production options exceeding 5 lb (2.3 kg) over the standard items they replace, not previously considered in curb weight or payload, including heavy-duty brakes, height-adjustable suspension, roof rack, heavy-duty battery, and special trim.

Recommended Inflation Pressure

The tire inflation pressure recommended by the vehicle manufacturer, shown on the tire placard.

Radial Tire

A pneumatic tire in which the cords extending to the beads are laid at 90 degrees to the centerline of the tread.

Rim

A metal support for the tire to which the tire beads are attached.

Sidewall

The part of the tire between the tread and the bead.

speed rating

An alphanumeric code assigned to a tire that indicates the maximum speed at which the tire can operate.

Traction

The friction between the tire and the road surface. Traction provided.

Tread

The part of the tire that contacts the road.

Tread Wear Indicators

Narrow bands, sometimes called "wear bars," that are visible across the tread of the tire when only 1/16 inch of tread remains.

UTQGS

The Uniform Tire Quality Grading system is an information system for tires that provides consumers with ratings for traction, temperature, and treadwear. Ratings are determined by tire manufacturers using government-prescribed test procedures. The ratings are molded into the tire sidewall.

Vehicle Capacity Weight

The number of designated seating positions multiplied by 150 lb (68 kg), plus the rated cargo and luggage load.

Vehicle Maximum Load on the Tire

The load on an individual tire due to the curb weight of the vehicle and optional equipment, plus the maximum number of passengers and cargo weight.

Vehicle Normal Load on the Tire

The load on an individual tire, determined by distributing to each axle its share of the curb weight, optional equipment weight, and normal passenger weight, and dividing by 2.

Vehicle Placard

A label permanently attached to the vehicle showing the original tire size and recommended inflation pressure.