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Tire Terminology

Air Pressure:          The amount of air inside the tire pressing outward on

                             each square inch of tire, expressed in pounds per

                             square inch (psi).


Alignment:             The state in which all wheels on a vehicle are pointed in

                             the optimum direction relative to one another.


All-Season Tires:     Tires that are designed for use on dry and wet

                             pavement and also provide traction on snow and ice.


Aspect Ratio:          The percentage relationship of a tires height to its

                             section width. The most common is 75, with the range

                             continuing downward (65, 60, etc.) to 35. Low aspect

                             ratio (or ‘series’) tires are also referred to as low



Balance:                 The state in which a tire and wheel spin with all their

                             weight distributed equally.


Bead:                     A round hoop of steel wires, wrapped or reinforced by

                             ply cords, that is shaped to fit the rim; it holds the tire

                             onto the rim.


Belt:                      A rubber-coated layer of cords that is located between

                             he plies and the tread. Cords may be made from steel,

                             fiberglass, rayon, nylon, polyester or other fabrics.


Casing:                  The tire body beneath the tread and sidewalls.

Cold Inflation         The amount of air pressure in a tire, measured in

Pressure:                pounds per square inch (psi), before a tire has built up

                             heat from driving.


Cord:                     The strands of fabric forming the plies or layers of the

                             tire. Cords may be made from steel, fiberglass, rayon,

                             nylon, polyester or other fabrics.


Contact Patch:        The portion of the tread that makes contact with the



Friction:                 The force between the tires and the road surface that

                             causes the tire to grip the road.


Groove:                 The space between two adjacent tread ribs; also called

                             tread grooves.


Highway Tires:       Also called Summer tires; designed for wet- and

                             dry-weather driving, but not for use on snow and ice.


Hydroplaning:         A floating effect caused by tires losing contact with a

                             surface covered with water.


Innerliner:              The innermost layer of a tubeless tire. The innerliner

                             prevents air from permeating through the tire.


Load Index:            An assigned number ranging from 0 to 279 that

                             corresponds to the load carrying capacity of a tire.


Maximum Inflation The maximum air pressure to which a cold tire may be

Pressure:                inflated, and the number is found molded onto the

                             sidewall. It is often different than the vehicle

                             manufacturers recommended operating pressure.


Overall Diameter:    The diameter of the inflated tire, without any load.


Overall Width:        The distance between the outside of the two sidewalls,

                             including lettering and designs.


Plus-Sizing:            An option allowing drivers to customize their vehicle by

                             mounting low-profile tires on wider rims of one or two

                             inches greater diameter, usually enhancing vehicle

                             appearance, handling and performance.


Ply:                       A rubber-coated layer of fabric containing cords that

                             run parallel to each other, extends from bead to bead

                             and goes between the innerliner and belts or tread.


Radial Ply Tire:       A type of tire with plies arranged so cords in the body

                             run at 90 degree angles to the center line of the tread.


Rim:                      A metal support for a tubeless tire or a tire and tube

                             assembly upon which the tire beads are seated. Often

                             referred to as wheel or wheel rim.


Rim Diameter:        The diameter of a tire rim, given in nearest whole

                             numbers (e.g. 15 inches).


Rim Width:             Distance between the two opposite inside edges of the

                             rim flanges.


Rolling Resistance: The force required to keep a tire moving at a uniform

                             speed. The lower the rolling resistance, the less energy

                             needed to keep a tire moving.


Rotation:                The changing of tires from front to rear or from side to

                             side on a vehicle according to a set pattern; it provides

                             even treadwear.


Section Height:       The height of a tire, measured from rim to the outer



Section Width:        The distance between outside of sidewalls, not

                             including any lettering or designs.


Shoulder:               The area of a tire where the tread and sidewall meet.


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


Sipes:                    Special slits in a tread that improve traction when the

                             road surface is wet or has dust, dirt, sand, snow or

                             other material on it.


Size:                      The combination of tire width, construction type, aspect

                             ratio and rim size used in differentiating tires.


Skid:                      To slip or slide on the road when tires lose their rolling



Snow Tire:             Often referred to as Winter tires, a special type of tire

                             with a tread and compound that gives better traction in

                             snow and other extreme winter conditions.


Speed Rating:         An alphabetical code (A.Z) assigned to a tire indicating

                             the range of speeds at which the tire can carry a load

                             under specified service conditions.


Tire:                      A precisely engineered assembly of rubber, chemicals,

                             fabric and metal designed to provide traction and

                             cushion road shock and to carry a load under varying



Tire Information     A metal or paper tag permanently affixed to a vehicle

Placard:                 that indicates the appropriate tire size and inflation

                             pressure for the vehicle, as well as rim size and load

                             capacity information.


Tread:                   That portion of a tire that comes into contact with the

                             road. It is distinguished by the design of its ribs and



Treadwear              Narrow bands, sometimes called ‘wear bars’, that

Indicator:               appear across the tread of the tire when only 2/32 inch

                             of tread remains.


Tread Width:          The width of a tires tread.


Traction:                The friction between the tires and the road surface, or

                             the amount of grip provided.


Uniform Tire           A tire information system that provides consumers with

Quality Grading       ratings for a tires traction (from AA to C) and for

(UTQG):                temperature (from A to C). Tread wear is normally

                             rated from 60 to 620. Ratings are determined by tire

                             manufacturers using government-prescribed test

                             procedures and are molded into the sidewall of the



Wear Bars:             See Treadwear Indicator.