Tire Terminology and Definitions

Air Pressure

The amount of air inside the tire pressing 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 additional accessories. Some examples of additional accessories are automatic transmission, power seats, and air suspension or 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 value of air pressure in a tire, measured in pounds per square inch (psi) or kilopascals (kPa) before the tire warms 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

A code molded into the tire sidewall indicating that the tire conforms to 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

Gross Axle Weight Rating, front axle.

GAWR RR

Gross Axle Weight Rating, 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 as primarily intended 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 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 maximum permissible load for 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 asymmetric tire has a specific side that faces outward when mounted on the vehicle. The outboard sidewall has white lettering or the manufacturer's brand and/or model name molded higher or deeper than the same moldings on the inboard sidewall.

Passenger (P-Metric) Tire

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

Ply

A layer of parallel rubber-coated cords.

Pneumatic Tire

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

Tire Options Weight

The total weight of installed production options weighing more than 5 lb (2.3 kg) over the standard items they replace, not previously included in curb weight or accessory weight, including heavy-duty brakes, ride levelers, roof rack, heavy-duty battery, and special trim.

Recommended Inflation Pressure

The tire pressure recommended by the vehicle manufacturer, as shown on the tire and loading information label.

Radial Tire

A pneumatic tire in which the multi-ply cords that extend to the beads are laid at 90 degrees to the tread centerline.

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 operate.

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 appear across the tread when only 1/16 inch of tread remains.

UTQGS

Uniform Tire Quality Grading Standards, a tire information system that provides consumers with ratings for traction, temperature, and treadwear. The ratings are determined by the tire manufacturer using government test procedures and 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 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, accessory weight, and normal occupant weight, then dividing by 2.

Vehicle Placard

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