Tire Terminology and Definitions

Air Pressure

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

Extra Weight

This refers to the total weight of optional equipment. Some examples of optional equipment are automatic transmission, power seat, and air conditioning.

Aspect Ratio

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

Belt

A rubberized layer of cords located between the plies and the tread. The cords may be made of steel or other reinforcing materials.

Bead

The tire bead contains steel wires wrapped with 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 tread centerline.

Cold Tire Pressure

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

Curb Weight

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

DOT Mark

The code molded into the tire sidewall indicates that the tire complies with the United States Department of Transportation Motor Vehicle Safety Standards. 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

Maximum load on the front axle.

GAWR RR

Maximum load on the rear axle.

Intended Outboard Sidewall

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

Kilopascal (kPa)

A metric unit of atmospheric pressure.

Light Truck (LT) Tire

A tire designated by the manufacturer primarily for use on 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 from 1 to 279 corresponding to the load-carrying capacity of the tire.

Maximum Inflation Pressure

The maximum air pressure to which a cold tire can be inflated. The maximum air pressure is molded on the sidewall.

Maximum Load Capacity

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 occupants in the vehicle is calculated as the number of designated seating positions multiplied by 150 lb (68 kg).

Occupant Distribution

Designated seating positions.

Outboard Sidewall

An asymmetrical tire has a specific side that faces outward when mounted on the vehicle. The outside sidewall molding has white lettering or has manufacturer, brand, and/or model name molding that is raised or deeper than the same moldings on the inside sidewall.

Passenger (P-Metric) Tire

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

Ply

A layer of rubberized parallel cords.

Pneumatic Tire

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

Tire Options Weight

The total weight of installed standard and optional equipment exceeding 5 lb (2.3 kg) over the standard items they replace, not previously considered in curb weight or the additional weight, including heavy-duty brakes, trailer levelers, roof rack, heavy-duty battery, and special trim.

Recommended Inflation Pressure

The vehicle manufacturer's recommended tire inflation pressure as shown on the tire placard.

Radial Tire

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

Rim

The metal support for the tire on which the tire beads are seated.

Sidewall

The part of the 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. The amount of traction provided.

Tread

The part of the tire that contacts the road.

Tread Wear Indicators

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

UTQGS

The Uniform Tire Quality Grading Standards are a tire information system providing consumers with ratings for traction, temperature, and treadwear. The ratings are determined by tire manufacturers using government test procedures. The ratings are molded into the tire sidewall.

Vehicle Load Capacity

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 plus accessory weight plus the maximum passenger 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, cargo weight, and normal passenger weight, and dividing by 2.

Vehicle Placard

This label is permanently attached to the vehicle and shows the original equipment tire size and recommended inflation pressure.