Glossary of Frequently Used Tire Terms

Below are definitions of frequently used tire-related terms. Boldface words within definitions indicate terms defined elsewhere in the glossary.

All Season. A tire that is engineered to perform year-round. All season tires are designated M+S, for Mud and Snow. They are rated by their manufacturers to be capable of generating traction in modest mud and light snow.

All Terrain Tire (AT). A tire for all-wheel-drive or four-wheel-drive vehicles, including pickups, crossovers and SUVs. An all terrain tire is used most commonly on roads but is strong enough to run off road, including on rocky trails, gravel, sand and mud. The tread is more aggressive than on strictly street tires. Some but not all AT tires are better in snow than street tires are.

All Wheel Drive. A system that sends power full-time to all four wheels of a vehicle to improve traction in all weather, especially foul weather. Most systems vary how much power is delivered to each wheel depending on whether that wheel is slipping or not. More slip means less power to maintain traction.

Aramid. A synthetic material used in cords and belts in some tires, often all-terrain and mud tires. Aramid is intended to strengthen in the sidewall, the tread or both, compared with other cords. It was originally used as a fabric in bulletproof vests.

Aspect Ratio. Using millimeters, the height of a tire sidewall from the wheel lip to the outermost part of the tread divided by the width of the tire. Generally speaking, the higher the performance, the wider the tire and the lower the aspect ratio are likely to be. The aspect ratio shows up behind the width in a tire size: In 215/60R16, the number 60 is the aspect ratio—that is, the height of he tire from the wheel to the outermost tread is 60 percent of the width.

Asymmetrical Tread. A design in which the inner and outer halves of the tread are different. Some asymmetrical tires have “this side out,” “outside” or “SFO” (side facing out) or a similar notation molded into the sidewall to ensure correct mounting and proper performance. If they are not directional, they can be rotated side to side. If they are directional as well as asymmetrical, they must be rotated front to back, same side, and mounted to roll in the prescribed direction.

Bias-Ply Tire. A tire in which the fabric cords are installed at an angle from bead to bead inside a tire. Few bias-ply highway tires are made today for the consumer market, although some tires for the classic-car market continue to be made with bias-ply technology. Eventually bias ply treads were reinforced by belts to make bias-belted tires. Replace a bias-belted tire with another bias-belted tire; never mix radial and bias tires on the same car.

Bead. The reinforced edge of the tire around its opening that is stretched over the rim and sealed to the wheel. The wheel size must match the bead size to prevent tire failure.

Belt. A flat, internal, circumferential component of a tire that reinforces the tread. The belt is inserted between the tread and the carcass when the tire is being built. It consists of plies usually made of woven cords surrounded by rubber. A belt does not extend to the beads. Touring tires have one or two belts. High-performance tires have two or three belts, more typically two belts with a cap ply between the belts and the carcass. Belts may be polyester for a smooth ride, nylon for a smooth ride but more support, steel for greater tread stability, or aramid for strength off road.

Bladder. The inner surface of a tubeless tire that seals it against air loss without an inner tube.

Block. A raised portion of the tread that is surrounded by voids both grooves and channels, and meets the road to generate traction.

Bridge. A raised part of the tire that connects two blocks and extends lower than the top surface of the tread. A bridge adds strength and stability, reducing squirm and wear.

Carbon black. A crystalline product of the combustion of heavy petroleum products that is an ingredient in tire manufacturing. Carbon black gives tires their color and reinforces the rubber compounds, helping the rubber to maintain its molded and cured shape. It also helps to dissipate heat that builds in tires while in operation.

Cap Ply. A belt of fabric cords, usually nylon, that sits between the reinforcing belts of a tread and the carcass. Mostly used in high performance and ultra high performance tires, cap plies discourage separation of the belts from heat caused by high speeds.

Carcass. The part of the tire that supports the weight of the vehicle and largely absorbs bumps. Sidewalls cover the outeward and inward sides of the carcass, and the tread covers the bottom. The bladder lines the entire inner surface.

Channel. Also, circumferential channel, circumferential groove or rain channel. Channels are the continuous voids that define the ribs on a tread. They help to prevent hydroplaning by moving water away from the tread surface.

Cord. A fabric reinforcement inside a tire that helps it retain its shape, provide elasticity and determine, to some extent, ride quality, stiffness and strength. Cords are like tiny cables, made from threads of fabric that are wound together, then encased in rubber. They also are used at the core of belts, which reinforce the tread to provide stability, reduce wear and enhance cornering.

Commonly used cord materials:

• Used primarily in the carcass to provide a smooth ride. Tires that use polyester cord in the tread as well are likely aimed at a smooth ride rather than better handling and are probably lower-priced tires.
• Also used in carcasses and treads, with less pliability and more strength than polyester. Often used for cap plies, which are built into a tire between the tread belt and the carcass, to discourage ply separation caused by heat on higher-speed tires.
• Used primarily in belts to keep the tread well planted in turns for better cornering, and in a straight line for more stability, less wear, and more even wear.
• Aramid. An extremely strong thread that is used primarily in off-road tires, especially mud tires, to stand up better to rocky terrain.

Cosmetic Performance Tire. An economical tire that has the looks of a traditional performance tire, such as a low aspect ratio and raised white letters, but has modest performance capabilities.

Crossover. A vehicle similar to an SUV that has a single cab enclosing the passenger and cargo areas, often with all wheel drive, and built on a unibody passenger car platform.

Department of Transportation, or DOT. The federal department that oversees highway, vehicle and tire safety. It also oversees the National Highway Traffic Safety Administration, which gathers and investigates tire complaints and, where needed, issues recalls.

Dimple. Any of a series of cylindrical indentations molded into a tread to promote cooling. Dimples are found on some ultra high performance tires.

Directional Tread. Also called a Directional Tire. A tire designed to roll forward in a specified direction. Directional tires usually have a small arrow on the sidewall pointing in the correct forward rolling direction. Directional tires must be rotated front to back on the same side.

DOT certified. DOT is the Department of Transportation. A tire that is DOT certified is one that has features the department considers necessary for use on public roads. Tires that lack the letters “DOT” on the sidewall, including most racing tires, are not legal on the street.

DOT Code. Numbers that reveal the date of manufacture for a tire, and the location. On the side of tires made since the year 2000, look for the letters DOT followed by a series of 10-12 letters and numbers ending in four numbers—for instance, DOT … 0216. That DOT code indicates that the tire was manufactured in the second week (02, or January), in the year 2016 (16). The two letters or numbers immediately following the letters DOT indicate the plant where the tire was made. This is important in recalls. To find out what each location means, consult the Tire Safety Group website.

Dual-Compound Tread. A tread with a dual-rubber compound typically uses a stronger, less resilient rubber above the tread belts and a more pliable rubber in the part of the tread that meets the road.

Extra Load. A tire that can be inflated to higher pressures. Extra Load tires often are LT (light truck) tires or high performance tires.

Four Wheel Drive. A system, usually activated by a switch or lever, that sends power to all four wheels to increase traction. Often equipped with a locking two-speed transfer case, four wheel drive is more effective in severe conditions than all wheel drive.

Groove. A lateral or diagonal void between blocks. Grooves, along with channels, form blocks, provide drainage and cool the tread. They also help to characterize the traction a tire generates.

High Performance Tire. A tire that provides stability, cornering power and  response at speeds of 149 mph and over. High performance tires usually have a speed rating of V (to 149) or ZR (over 149). They may not be suitable for use in near-freezing and colder weather unless they are M+S rated.

Hub. The part that connects the wheel to the axle on a vehicle. It contains a wheel bearing to reduce the friction caused by rolling and a brake to slow and stop rolling. A hub has threaded pins called studs that fit through holes in the wheel and accept lug nuts to fasten the wheel tight.

Hydroplaning. The undesirable characteristic of some tires to ride up onto the surface of rain or standing water at speed, causing a dangerous loss of traction. Engineers design grooves and channels to siphon water away from the tire’s surface to prevent hydroplaning.

Jack. A device for safely raising and lowering a vehicle to replace a wheel and tire. Not all jacks fit all vehicles, so if your vehicle lacks a jack, purchase one that fits. Dealers are a good source of new jacks. Salvage yards are a good source of less expensive used jacks. Either can be found online with a Google search.

Load Range. A letter used to express sidewall strength. Most sidewalls are 2 plies of corded rubber, with some performance and truck tires 3 plies and higher. Today’s tires are rated on equivalent strength, such as “4-ply rating”or “10-ply rating,” not the actual number of plies. The deeper into the alphabet the letter, the stronger—and probably the stiffer—the sidewall.

Common Load Range designations:

B—4-ply rating (most passenger tires)

C—6-ply rating

D—8-ply rating

E—10-ply rating

F—12-ply rating

Light truck. Tires designated LT are highly suited to towing and commercial use on vans, SUVs and pickups. They are usually Load Range C or higher.

Lug. 1. Any of several threaded metal studs that protrude from a hub and fit through the holes in a wheel, which is fastened onto the studs with lug nuts. 2. A big, aggressive block formed by extra wide voids on a mud tire or an all terrain tire.

Lug nut. The hexagonal fastener that screws onto a threaded stud or lug to attach wheels to hubs.

Lug Wrench. A tool, usually in the form of an “X,” that is used to loosen and tighten the lug nuts that fasten a wheel to a hub. Each lug wrench end fits a different size nut, but not all wrenches have the same sizes. Some jack handles double as a lug wrench. Make sure the wrench you buy fits the lug nuts on your vehicle and stow it in your trunk or cargo area.

M+S. Also M&S. Mud and Snow. A tire with this rating is said by the manufacturer to have a tread design that can generate traction in modest mud and light snow. All season tires are rated M+S.

Metric. A system of measurement that begins with the tire’s width in millimeters—for example, 215/60R16. A metric tire can handle a higher maximum load than a P215/60R16 P-metric tire, although they are an identical size. Therefore, you can replace a P-metric tire with a metric tire, but you should not replace a metric tire with a P-metric.

National Highway Traffic Safety Administration.  An agency within the Department of Transportation. It gathers and investigates vehicle and tire complaints and, where needed, issues recalls.

New Tire. A tire that has never been mounted. New tires sold by BestUsedTires.com are covered by a 90-day return policy from VYC Tires, Inc., owner of bestUsedTires.com. New tires sold by BestUsedTires.com do not carry manufacturer warranties.

Notch. An indentation of varying length in a tread. Unlike a groove, a notch does not completely intersect a rib to form individual blocks. A notch may be straight, curved or angled, and it may be horizontal, vertical or diagonal. It helps to drain water from the tread surface and increase foul-weather traction.

Off-Road Tire. A tire engineered to operate off pavement on an all-wheel-drive or  four-wheel-drive vehicle. Less capable but more comfortable are all terrain tires, which are quieter and better riding on pavement but can handle soft roads, modest mud, light snow, gravel, and tamer trails and fields. More capable but less comfortable are mud tires, which are noisier and rougher-riding on pavement but can handle crude surfaces, including gravel, sand, deep mud and rocky terrain, and in some cases, deep snow. Mud tires sometimes can accept metal studs for extra traction on ice.

OWL. Outline White Lettering. Letters on the sidewall denoting the tire brand and model name, and sometimes other information, such as size, stand out from the sidewall and appear in letters that are white with a black fill.

Patch. An internal application over a puncture that repairs the bladder so it will again maintain pressure. Tread punctures can be patched; sidewall punctures cannot be repaired.

P-metric. A system of tire measurement that begins its designation with the letter “P.” Because P-metric tires have a lower load index than a metric tire—that is, they can handle a lower maximum weight—never replace a metric tire with an otherwise identically sized P-metric tire. You can, however, replace a P-metric tire with a metric tire, since the metric tire can handle more weight.

Performance Tire. A tire that’s engineered for high stability in both straights and corners, with higher cornering grip and quicker response. It usually has a speed rating of H (130 mph) or V (149).

Plug. A repair method that fills and seals a tread puncture so the bladder will again maintain air pressure. Sidewall punctures cannot be repaired safely.

Puncture. A wound that penetrates the bladder of a tire, such as from a screw or nail, causing pressure loss. Many but not all punctures in the tread can be repaired with a plug or patch. Sidewall punctures cannot be repaired safely.

Radial Tire. A tire in which the plies containing fabric cords are installed perpendicularly from bead to bead inside a tire. Most of today’s tires are radials. They last longer and give better fuel mileage than a bias-ply tire. Replace a radial only with another radial. Never mix radial and bias-ply tires on the same vehicle.

Recreational Vehicle. A vehicle designed with full living support, including a kitchen and bathroom, sleeping accommodations, and hookups for water, electric, and septic. Recreational vehicles may be a trailer, which uses ST tires, or a motorhome, which usually uses LT tires, which are truck tires.

Response. The ability of a tire to quickly do what the driver’s steering movements tell it to do.  Tires with a solid or semi-solid central rib tend to have more responsive handling. They deliver more “feel” to the driver.

Rib. A circumferential row of blocks or a solid raised circumferential portion on a tread. Solid ribs tend to enhance handling at the center of a tread and stability at the shoulders.

Rolling Resistance. The energy required to move a tire. A tire with a low rolling resistance requires less energy from the vehicle to move it, so it uses less fuel. Low rolling resistance tires also are called fuel savers.

Run Flat Tire (RFT). Sometimes called Run-On-Flat (ROF). A tire with a thicker, stiffer sidewall that will support the tire for up to 50 miles after pressure is lost. It can be driven without pressure at speeds up to 50 mph. It is preferred on vehicles that have no space to accommodate a spare tire.

RWL. Raised White Lettering. Letters on the sidewall denoting the tire brand and model name, and sometimes other information, such as size, and stand out from the sidewall and appear in letters that are white.

Self-Cleaning Tread. An engineering element of many off-road tires and some winter tires. A self-cleaning tread expels stones and built-up mud and snow. Self-cleaning often is accomplished by molding in a slightly raised shape at the bottom of grooves and rain channels.

Self-Sealing. A tire that internally heals over punctures to prevent pressure loss. A self-sealing tire has a bladder that will snug around a screw or nail to slow, and possibly stop, air loss.

SFI. Side facing in. This indicates the side of a tire that must be mounted to the inside on asymmetrical tires.

SFO. Side Facing Out. This indicates the side of a tire that must be mounted to the outside on asymmetrical tires.

Shoulder. An outer rib of a tire tread.

Sidewall. The two sides of the carcass between the tread and the bead. Sidewalls have cord reinforcements inside, usually polyester or nylon, that absorb the energy of road irregularities but return to their shape. The sidewalls are aided in their work by pressurized air inside the tire, held intact by the bladder. In run flat tires (RFT) or run on flat tires (ROF), the sidewalls are strong enough to enable a tire to run up to 50 miles at 50 mph after pressure is lost. Such tires usually have a somewhat stiffer ride when pressurized than similar tires that are not RFTs. Sidewalls may have nonstructural, decorative elements, such as raised white letters and whitewalls. The white features are made from rubber that is not colored by carbon black and are not structural.

Sidewall puncture. A tire wound that penetrates the sidewall of tire, causing pressure loss. Never repair a sidewall puncture; replace the tire.

Sidewall tread. Tread elements designed to deliver traction where the sidewall of a tire meets an irregular surface. Sidewall tread elements are often present on all-terrain and mud tires.

Silica. A material found in sand that is added to rubber compounds to keep tires more pliable, especially in cold weather, without making them softer. Tires with silica in the tread tend to perform better in dry, wet and snowy weather, and in cold temperatures. They also last longer.

Sipe. Any of the narrow slits molded into the tread of a tire. Sipes come in any of several shapes, including curved, straight and zigzag. Sipes can run in any of several directions, including lateral, vertical, diagonal, angled or curved. They can also vary in depth, from just part of the block or tread depth, which means they disappear as the tire wears, to full depth, which means they remain effective even as tread nears the end of its life. Sipes increase a tire’s bite in any weather. Sipes are especially important in all-season tires, and even more so in winter tires, where zigzag sipes multiply the number of edges that grip ice and packed snow. Sipes also help to quiet tires on pavement.

Snow Tire. Also called a Winter Tire. A tire that has a tread capable of gripping deep snow, packed snow and glare ice. Many blocks with bigger-than-average voids help to grip deep snow. Ample sipes and a tread compound that remains pliable in freezing weather help to grasp packed snow and ice. Some accept metal studs to increase traction on packed snow and ice, but usually produce more road noise and ride rougher than tires without studs.

Soft Road. A gravel road, usually well packed from use, that is more drivable than a trail, which may be more rutted, rockier and muddier. Many SUV tires that are primarily on-street tires can travel soft roads but not off road.

Solid rib. A circumferential row in the tread that is not broken into blocks. Some ribs that have long, deep notches are considered semi-solid. A solid rib imparts response in the center of the tire and stability on the shoulders.

Spare Tire. A fifth tire carried in or under the trunk or cargo bay, or on a mount atop or behind some SUVs. It can be used in place of a flat tire. Whether a temporary spare or a conventional tire, a spare should be mounted permanently on a wheel so it can be mounted on a vehicle on the side of a road or at a service station. A jack and a lug wrench are required to fit a spare tire to an axle. The wheel must fit the lug pattern. Make sure your car has a spare; not all new cars do, and spares sometimes are missing from used cars.

Speed Rating. A letter indicating a manufacturer’s assertion that a tire is capable of safely meeting a certain speed. Tires are tested on machines to simulate the maximum speed, either for 10 minutes at 71 degrees (European method) or one hour at 100 degrees (SAE, or Society of Automotive Engineers, method).

Common speed ratings:

N=87 mph (commercial truck tires)

P=93 mph (commercial truck tires) 

Q= 99 mph (some commercial, a few consumer truck tires)

R=106 mph (some commercial, a few consumer truck tires)

S=112 mph

T=118 mph

U=124 mph

H=130 mph (performance)

V=149 mph (high performance)

ZR=over 149 when it appears in the size designation (high performance); over 186 when it appears in the size designation and is followed by (Y), including the brackets (ultra high performance)

W=168 mph (ultra high performance)

Y=186 mph (ultra high performance)

(Y)=above 186 mph (ultra high performance)

Squirm. An undesirable tendency in a tire to subtly rub in multiple directions against the road surface, accelerating wear and eroding control.

Special Trailer. ST tires are made specifically to handle the stresses and load characteristics to which a trailer tire is subjected. Tires in this class often are bias-ply but increasingly are radials, which are required on many new recreational vehicles. Bias and radial tires should not be mixed on the same trailer, but bias tires can be used on a trailer that’s towed by a radial-equipped truck.

Stud. 1. Any of the threaded metal projections on a hub that go through the holes in a wheel and accept lug nuts to fasten the wheel to the hub. 2. Any of the metal pins that can be mechanically inserted into receptacles molded into some winter tires to enhance traction on ice and packed snow. Some states ban studded tires and others limit the number of months the tires can be used because of the extra wear they cause to paved surfaces. Check before buying.

Summer Tire. A tire engineered for use in warmer weather. Summer tires should not be used in near-freezing and colder temperatures or in snow. They should be replaced by all-season tires or winter tires in cold weather.

SUV. Sport Utility Vehicle. A vehicle often built on a full frame that has all wheel drive or four wheel drive, with a single cab that encloses the passenger and cargo areas. Some newer SUVs are built on unibody chassis.

Symmetrical Tread. A design in which the outer and inner halves of the tread are identical, but sometimes with the direction of blocks in each half running in the opposite direction. Symmetrical treads may or may not have a central rib. Symmetrical tires that are non-directional—that is, that must not roll in a specified direction—can be rotated to the opposite side.

Temperature Rating. A letter representing a tire’s ability to disperse heat during operation. “Temperature A” is the best rating, stepping down to B and C. Generally speaking, with all other things being equal, a tire with a higher traction rating is likely to have more durability than a tire with a lower rating. The rating is part of the government-required UTQG.

Tire Pressure Monitoring System. A wirless system in which sensors are amounted inside the wheel and connected to the valve stem.  A sensor triggers a warning light on the instrument panel if it detects low pressure.

Tire Rotation. The practice of periodically moving tires on the front of the car to the rear, and vice versa, usually every 6,000 miles or twice a year. Most tires can be rotated from side to side, but directional tires must be rotated on the same side.

Tire size. Expressed in this order: width, height, type of construction and wheel diameter. In a tire labeled 215/60R16, the tire:

• Is 215 millimeters wide
• Has a height from the bead to the outer tread that is 60 percent of the width
• Is a Radial tire
• Mounts on a 16-inch rim.

Traction Control. A system required on all new light trucks and passenger vehicles sold in the U.S. to increase traction. Most systems use the antilock brake system to rapidly and lightly cycle a brake on and off so a drive wheel that slips will be able to grip a slick or loose surface.

Tread. The collected system of raised elements on a tire that are designed to generate traction as they meet the road surface. Tires must have a tread to be approved by the Department of Transportation for street use.

Tread Wear Grade. An abstract number that reflects a manufacturer’s tests of a tire’s durability. In general, the higher the number, the longer the tire is likely to last in terms of miles traveled. A tire stamped “Tread 600” is estimated to have twice the durability of a tire stamped “Tread 300,” but neither number reflects the expected number of miles it will travel before being worn. It is part of the government-mandated UTQG.

Touring Tire. A tire that excels at highway cruising with a smooth ride, predictable handling and long wear. Most tourers sold in the U.S. are all-season tires, but some are summer tires.

TPMS. See Tire Pressure Monitoring System.

Traction Rating. A letter representing the ability of a tire to stop in a straight line on dry pavement. “Traction AA” is the best rating, stepping down to A, B and C.

Temporary Spare. Sometimes called a “doughnut spare.” A narrow tire mounted on a specially sized wheel that can substitute for a regular tire that goes flat. Temporary spares, which are carried in or under the trunk, should be driven no more than 500 miles at no more than 50 mph before being replaced by a permanent tire and wheel. Not all temporary spare wheels fit all vehicles.

Tuner. An owner who modifies the look and performance of his vehicle. Tuners often use upsizing to achieve a custom wheel and tire look.

Ultra High Performance. A tire that can deliver traction, response and cornering capabilities at very high speeds. Ultra high performance tires usually carry a speed rating of W (168 mph), Y (186) or ZR (Y) (also 186). Unless rated M+S, they seldom are suitable in near-freezing and colder temperatures. Street legal, they also are highly suitable for track days and timed events.

Upsizing. The process of installing wider tires with a lower aspect ratio on bigger-diameter wheels, preferred by tuners. The outer diameter or height of the upsized mounted tires should not exceed the outer diameter or overall height of the mounted original tires. Significant changes would distort speedometer readings and might cause tires to rub against a vehicle’s bodywork. To calculate the height of the tires, divide the width by the aspect ratio and multiply by 2, then add wheel diameter in millimeters rather than inches.

Used Tire. Any tire that has been mounted on a vehicle, then removed and resold. Used tires do not carry a manufacturer’s warranty. VYC Tires, Inc., which owns BestUsedTires.com, does provide a 90-day return policy. BestUsedTires.com pressure-tests every tire it sells. Used tires cost a faction of what a new tire costs. Used tires are a good solution for replacing one worn tire or two from a set that is no longer manufactured, and for reliable but lower-cost tires.

UTQG. Uniform Tire Quality Grade. A government-required assessment of a tire’s capabilities and durability. It includes a Tread Wear Grade, Speed Rating, Temperature Rating, and Traction Rating. Manufacturers assign the ratings after conducting tests using federal guidelines

Variable Pitch Tread. A combination of changes in tread block shape, size and angle that control harmonics. Variable pitch treads tend to be quieter at speed or produce a combination of noises that are less offensive to drivers and passengers.

Void. The hollow space formed by channels and grooves around blocks and between ribs on a tread. Wider voids make a tire more aggressive for off-road use and for water management. Narrower voids tend to make a tread quieter. Intermediate widths are typical of all-season tires.

Winter Tire. Also called Snow Tire. A tire that has a tread capable of gripping deep snow, packed snow and glare ice. Many blocks with bigger-than-average voids help to grip deep snow. Ample sipes and a dual compound tread that remains pliable in freezing weather help to grasp packed snow and ice. Some accept metal studs to increase traction on packed snow and ice, but usually produce more road noise and ride rougher than tires that are stud free.