Tire Glossary

Air Pressure The amount of air inside the tire pressing outward on each square inch of its surface; expressed in pounds per square inch. For cars and light trucks, the air pressure measured when the tire is cold (before the vehicle is driven) establishes its recommended air pressure setting. Alignment The mechanical condition of adjustable components within the vehicle’s suspension. When a vehicle is in alignment, the caster, camber, toe-in and thrust settings are set to specification. Severe impacts (hitting potholes or curbs) and worn suspension parts are the leading causes of misalignment. All-Season Tires Tires designed to provide good traction in a wide variety of road conditions, including wet, dry and mud and snow. This design also limits the tire’s performance in extreme conditions, or when compared to tires built for a particular category. Alphanumeric A term for describing the size of a tire (H78-15, for example) where both letters and numbers are used.
Aspect Ratio A term that describes a tire’s height-to-width proportion. If a tire’s sidewall height were 65% of its section width, its aspect ratio would be 65. In the tire size expressed as 205/65-15, the number 65 is the aspect ratio.
Balance The state in which a tire and wheel assembly spins with all its weight distributed equally. A wheel balancer is used to place weights compensating for static and dynamic imbalances that exist in all assemblies. Not balancing an assembly can result in vibration. (Tire Balancing article)
Bead A round hoop of steel wires, wrapped or reinforced by steel cords, placed at the very inside of the tire’s diameter. (Tire Construction article)
Bias Ply Tire A pneumatic tire manufactured such that the plies are laid at alternate angles less than 90 degrees to the centerline of the tread. These criss-cross plies give the tire its strength, but generate heat during operation and limit the tire’s wear and performance.
Carcass The tire body beneath the tread and sidewalls; also called the casing.
Contact Patch The portion of the tread that contacts the road during operation. Cord The strands of material forming the plies or layers of tire. Cords may be made from fiberglass, rayon, nylon, polyester or steel. DOT Markings Each tire has a required Department of Transportation number imprinted on at least one of its sidewalls. That number begins with the letters "DOT" and may contain up to 12 additional numbers and letters. The first and last digits are the most important for the tire owner. The first two letters/numbers identify the manufacturer of the tires. Prior to the year 2000, the last three digits of a DOT number represented the week (two digits) and the year (one digit) of production. For example, if the last three digits are 439, the tire was produced in the 43rd week of 1999. Tires produced after January 1, 2000 have a four digit date code at the end of the DOT number. The first two digits represent the week of production and the last two digits represent the last two digits of the year of production. So, 3500 as the last four numbers indicates that the tire was produced in the 35th week of the year 2000. Footprint The portion of the tire that makes contact with the surface of the road. Friction The resistance of one material (the tire tread) as it moves against another (the road); this is the force that causes the tire to grip to the road. Gross Vehicle Weight (GVW) The actual weight of a vehicle when fully loaded with passengers and cargo.
Groove The space between two adjacent tread ribs; also called tread grooves.
Heat Cycling A method of "breaking in" competition tires prior to initial use. Heat cycling gradually heats the tire in a controlled environment to gently stretch the tread compound, resulting in better traction and longer tread life. Highway Tires Also called summer tires; designed for dry and occasional wet weather driving, but not for use on snow and ice. Hydroplaning A skimming effect caused by tires losing contact with a surface covered by water. Innerliner The innermost layer of a tubeless tire which prevents air from permeating through the tire. This thin layer of material replaces the innertube. Load Index An assigned number ranging from 0 to 279 that corresponds to the load carrying capacity of the tire. (Load Index chart) M+S, M/S or M & S (Mud and Snow) Indicates that a tire can reach particular standards for performance in mud and snow conditions. The tire must meet the Rubber Manufacturers Association (RMA) definition of a mud and snow tire. Maximum Inflation Pressure The maximum air pressure to which a cold tire may be inflated; found molded onto the tire’s sidewall. OE and OEM OE means "Original Equipment" and refers to the tires included with a new vehicle at the time of purchase. The vehicle's manufacturer selects these tires to provide the optimal performance based on the performance characteristics of the vehicle. "OEM" stands for "Original Equipment Manufacturer." Overall Diameter The diameter of an inflated tire without any load. Overall Width The distance between a tire's outside sidewalls, including lettering and designs. P-Metric Uniform designation of tire sizes in metric measurements originally introduced by American tire manufacturers in 1977. Commonly called "P-metric series." A typical P-metric tire size is P215/70R-15. Placard A small label typically located on the edge of the driver's door or inside the glove compartment of a vehicle. A placard contains information on the vehicle such as the manufacturer's recommended tire inflation pressure, seating capacity, and Gross Vehicle Weight (GVW).
Plus-Sizing An option allowing drivers to customize the appearance and performance of their vehicle by mounting lower profile tires on larger diameter wheels. One-inch greater wheel diameter is referred to as plus-one, two inches is plus-two... and so on. Using a lower profile tire with a greater diameter rim allows the overall diameter to remain about the same. (Plus Sizing article)
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 of tread.
Ply Rating This letter indicates the load carrying capacity of the tire in terms of its construction. A "C" indicates the tire has a 6-ply load carrying capacity. The tire is not actually built with 6 plies, but contains one or two plies of equivalent strength. A "D" is an 8-ply rating, and an "E" is a 10-ply rating. If there is no letter, the tire has a standard 4-ply rating. PSI Pounds Per Square Inch. This is the standard unit of measurement for air pressure within tires. Radial Ply Tire construction where the cords in the body run at 90 degrees to the centerline of the tread. Ride Matching A system of balancing a tire and wheel assembly using a simulated road test. Ride matching provides optimal weight distribution and eliminates vibrations caused by the combination of minor errors within tires and wheels. (Ride Matching article)
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 constant speed. The lower the rolling resistance, the less energy needed to keep a tire moving.
Rotation Moving tires from side to side or front to rear on a vehicle in a prescribed pattern to achieve uniform wear on all tires. Rotations should be performed regularly every 6,000 miles. (Tire Rotation article)
Section Height The height of a tire measured from the rim to the outer tread. (Reading the Tire Size article) Section Width The distance between outside sidewalls, not including any lettering or design. (Reading the Tire Size article)
Series A numerical representation of a tire’s aspect ratio. For example, 60 Series indicates the tire’s section height is 60% of its section width (See Aspect Ratio). Shimmy Wobbling of wheels from side to side on a vehicle. Improperly balanced tires, misalignment and bent wheels can cause shimmying.
Shoulder The part of a tire where the sidewall and tread meet. Certain tire design features shoulder blocks for better traction.
Sidewall The part of the tire between the tread and the bead.
Size An expression that defines a particular tire in terms of its width, height, rim diameter, aspect ratio and construction type. 205/65R-15 expresses tire size using the metric system. For more detailed information, visit our page on reading the tire size. (Reading the Tire Size article) Snow Tire Also referred to as a winter tire; a special type of tire with a tread and compound that gives better traction in snow. Identified by M&S, M+S or M/S on the sidewalls. All season tires also include these designations on the sidewall. (Winter Tire FAQ) Speed Rating The speed rating of a tire is based on U.S. Government standards for reaching and sustaining a specified speed. Typically, a tire with a higher speed rating results in better handling. Speed ratings are determined via laboratory tests that simulate road performance at various speeds. Tires are assigned a single letter (such as H or V) to designate speed rating. (Speed Rating article) Tire Pressure Monitoring System (TPMS) TPMS is an automated system that monitors the air pressure in a vehicle's tires. When air pressure in one or more tires drops 25 percent or more below the correct pressure, a warning alerts the driver. Traction The friction between a tire and the road surface; the amount of grip provided.
Tread The part of the tire that comes into contact with the road. The tread type is distinguished by the design of its ribs and grooves. (Tire Construction article)
Tread Depth The distance measured in the major tread groove nearest the centerline of the tire from the base of the groove to the top of the tread. According to law, most states legally consider a tire to be worn out when it reaches a tread depth of 2/32".
Treadwear Indicator Narrow bands, sometimes called “wear bars”, that appear across the tread when 2/32” of tread remains.
Tread Width The width of a tire's tread. UTQG (Uniform Tire Quality Grading) A tire information system that provides consumers with ratings for a tire's traction (AA to C) and temperature (A to C). Treadwear is a numeric rating. Ratings are determined by tire manufacturers using government-prescribed test procedures, and are molded into the tire's sidewall. These ratings can only be compared within specific manufacturer's tires and cannot be compared from one manufacturer to another. Our treadwear, traction and temperature page explains this rating system in much more detail. (Treadwear, Traction, and Temperature article) Valve A device mounted in the wheel that lets air in or out of the tire. Valves include caps to keep out dirt and moisture and a valve to prevent air from escaping. Source : (http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireTerm.jsp)