Tire Terminology and Definitions

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

Accessory weight: the total weight of additional accessories. Some examples of optional accessories are an intelligent variable transmission, power seats, and air suspension air conditioning.

Aspect ratio: the ratio of tire height to its width.

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

Bead: the bead of the tire contains steel wire wrapped in steel cables that hold the tire on the rim.

Bias ply tire: a pneumatic tire in which the plies are laid alternately 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 kilopascals (kPa) or pounds per square inch (psi), before the tire has built up heat from driving.

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

DOT marking: the DOT code includes the Tire Identification Number (TIN), an alphanumeric code that can also identify the tire manufacturer, manufacturing plant, brand, and date of manufacture.

GVWR: gross vehicle weight rating.

GAWR FRT: front axle gross axle weight rating.

GAWR RR: rear axle gross axle weight rating.

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

Kilopascal (kPa): metric unit of air pressure.

Light truck (LT) tire: a tire designated by its manufacturer primarily for use on light trucks or multipurpose passenger vehicles.

Load index: an assigned number in the range from 1 to 279 that corresponds to the load capacity of the tire.

Load ratings: the maximum load the tire is designed to carry at a given inflation pressure.

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 rating 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 passenger weight: the number of passengers the vehicle is designed to seat, multiplied by 68 kg (150 lb).

Occupant distribution: the seating positions assigned to occupants.

Outward-facing sidewall: the side of an asymmetric tire that has a specific side designated to face outward when installed on the vehicle. The outboard sidewall has white lettering or the manufacturer's brand, trademark and/or model name moldings that are higher or deeper than the same moldings on the inboard sidewall.

P-metric (passenger) 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 which, when mounted on a vehicle wheel, provides traction and contains gas or fluid that supports the load.

Variety of equipment: the combined weight of installed factory options and accessories exceeding the standard equipment by more than 5 lb (2.3 kg). Items that replace standard equipment are not previously included in curb weight, or additional weight, including heavy-duty brakes, ride height leveling devices, roof rack, heavy-duty battery, and special trim.

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

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

Rim: A metal support for the tire on which the tire beads are seated.

Sidewall: The portion 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. Provides the degree of grip.

Tread: The portion of the tire that contacts the road.

Tread wear indicators: narrow bands, sometimes called "wear bars," that appear across the tire tread when only 1.6 mm (1/16 inch) of tread remains.

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 tire manufacturers using government-specified test procedures. Ratings are molded into the tire sidewall.

Vehicle weight: the number of seating positions multiplied by 68 kg (150 lb), plus the calculated cargo and luggage load.

Vehicle Maximum Load on the Tire: The load on an individual tire due to curb weight plus optional weight plus the maximum number of passengers and cargo weight.

Vehicle Normal Load on the Tire: The load on an individual tire, determined by distributing the curb weight, optional weight, and normal passenger weight and 2 driver weights to each axle.

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