Tire Terms And Phrases Glossary

We admit, the specialized language in the tire world can be cumbersome and confusing. To take angst out of the customer-tire specialist conversation, here are some words and phrases you may encounter during the tire-buying process, from aquaplaning to zero offset. 

Air Pressure: The force exerted by air within a tire, shown most commonly in pounds per square inch (PSI) or kilopascals (KPs).

Alignment: Adjusting a vehicle’s wheels, steering and suspension parts to achieve the most efficient operation of all tire/wheel combinations as they relate to vehicle control and tire wear. When wheels are in alignment, they are in the optimal position relative to the road and each other. Misalignment comes when hitting a pothole or “curbing” a wheel.

All Season Tires: Tires built to perform well in varied road conditions throughout the year. An all-season tire must meet specified Rubber Manufacturers Association (RMA) criteria to be effective. All-season tires have an MS, M/S or M&S mark on the sidewall, meaning that they are suitable for use in mud and snow.

All-Terrain Tires: Tires designed with greater open space between tread lugs to offer increased traction through difficult or rugged terrain.

Approved Rim Width: The distance between the two inside edges of a wheel’s rim flanges on which a tire is mounted.

Aspect Ratio: Numerical term that expresses the ratio between the section height of a tire to its width. An aspect ratio of 55 means the tire section is approximately 55% high as it is wide. It will be expressed like 245/55ZR18. The aspect ratio is 55.

Asymmetrical: A tire design where the tread pattern on inner side of the tire differs from the tread pattern on the outer side.

Aquaplaning or Hydroplaning: When water on the road can’t be evacuated fast enough from beneath the tires, it causes a vehicle to ride atop the water. As a result, even though a driver might feed input to a steering wheel, the vehicle will not respond but instead will “aquaplane”.

Balancing: Adjusting the mounted tire/wheel package so it spins with evenly distributed weight. Tire technicians attach weights to wheels to correct imbalances.

Bead: The tire section that rests on and holds a tire to a wheel rim under inflation. The bead also refers to a hoop of high-tensile steel wires that forms the core of that portion of the tire.

Belt: Layers of cords wrapped in rubber that run circumferentially around a tire between the tread and the body plies. Belt cords are usually made of steel, but can also include fiberglass, polyester or nylon.

Bias Ply: A tire with body ply cords that run at an acute angle to the circumferential centerline of a tire.

Camber: A suspension setting that indicates the degree of a wheel’s inward or outward tilt from vertical (think pigeon or duck-toed, in and out). The angle is adjusted to keep the outside of each tire flat to the ground while “under load”, cornering or turning.

Carbon Black: An oil-based reinforcing filler material in tire rubber compounds.

Carcass and/or Casing: Supporting structure beneath a tread and sidewalls, Casing usually means it still has belts (as used by the retread industry); a carcass generally refers to a tire that’s worn down to its plies without belts.

Centerline: An imaginary line drawn circumferentially around a tire on the tread surface at the center or the tread. This is important when determining wheel camber settings.

Construction: How the plies of a tire are assembled. If you see the letter “R” on a tire, that connotes it is of a radial construction – the body plies run radially across the tire. Bias-ply construction, which carries a “B” or “D”, means plies run diagonally across the tire.

Contact Patch (or Footprint): The tire’s tread area that is in contact with the road while driving. The only thing between you and the asphalt.

Cord: Twisted fibers in plies or belts to provide strength. Cords are made from fiberglass, rayon, polyester, nylon or steel.

Deflection: When a tire goes through a corner at speed, pressure – loads – are transmitted through the tire. The deflection is the difference between the unloaded and loaded tire section tire height.

Directional Tread: A tire tread pattern designed to give improved performance when all are rolling in the same direction. These patterns are usually arrow or V-shaped and are used primarily in high performance and winter-tire designs; when they are rotated, they move only from front to back and vice versa.

DOT Number: The DOT number is an alphanumeric code on a tire’s sidewall that shows a tire complies with U.S. Department of Transportation motor vehicle safety standards. From this code you can determine in which factory the tire was made as well as size, type, and the week and year of manufacture.

Flat Spot: Irregular wear in an isolated area of a tire’s tread. Race drivers are known to “flat spot” a tire when they are under severe braking and the tire doesn’t rotate but instead scrubs along a road surface.

Grooves: Spaces between two adjacent tread ribs or “lugs”.

Handling: When talking about tires, handling describes the responsiveness of the tire to steering-wheel inputs by the driver.

Light Truck Tires: Designed for commercial light trucks, pickups or passenger vans. Designated with “LT” in the sizing information. Most consumer-oriented pickups, SUVs and vans are equipped with larger-sized passenger-designated tires, sometimes with an “XL” designation indicating extra load-carrying capacity.

Load Index: The maximum load that a tire can carry when properly inflated, expressed as a number from 70-126 for most car and light truck tires. LT-designated light truck tires are marked with two indexes, one for single tire use and one for dual-tire applications.

Load Carrying Capacity: The maximum weight a tire can carry under ideal conditions.

Low-Profile Tire: A tire whose cross section is squat in appearance – it has a narrow sidewall; its section width is noticeably wider than the section height. Generally considered effective for high-performance tires and as such its ride will be rougher as there’s less sidewall deflection in the tire.

Max Load Tire: The maximum weight a tire can support at its rated inflation pressure.

Micro Grooves (Sipes): Grooves in the tread blocks that increase driving stability, towing and braking, and improve water evacuation and snow traction between the tire and road surfaces. Sipes can run circumferentially, laterally or diagonally.

Mounting: The process of installing a tire onto a wheel.

M+S, M/S, M&S: Mud and snow – a designation that appears on all-season tire sidewalls, denoting the tire meets the Rubber Manufacturers Association’s definition of a mud and snow tire.

Negative Camber: Alignment where, when looking straight on at a car, the tires angle in toward the center of a vehicle. This alignment is often used on race cars to improve cornering grip.

Negative Offset: When a wheel-mounting surface is toward the back, or brake-side of the wheel. In a negative-offset wheel, the tire and wheel are moved outside of the vehicles wheel well.

Nitrogen: An inert gas (scientific designation “N2”) offered by some tire retailers in place of compressed air due to its larger molecule size that reduces the normal rate of pressure loss and is considered less corrosive to a tire’s steel belts.

Offset: Distance from the wheel mounting surface to the wheel centerline.

Original Equipment Fitment: Original equipment (O.E.) tires are those fitted to a new vehicle at purchase. These are selected by vehicle manufacturers to help provide optimal performance based on a vehicle’s characteristics. When time comes, some 37 percent of buyers choose the same tire that came with the vehicle when new.

Overall Diameter: An inflated tire’s diameter, less load, is twice the tire-section height, plus the normal rim diameter.

P-Metric: A tire with a “P” preceding its size designation shows it is for passenger cars.

Performance Tires (High Performance and Ultra High Performance): Tires that offer superior handling, grip and cornering ability compared with standard tires. They’re rated for operation at higher speeds.

PSI: The measurement of tire-inflation pressure. Pounds per square inch.

Ply: A layer of rubber-coated parallel cords that form a component of the tire casing.

Plus-Sizing: Adding larger diameter wheels to lower profile tires in an attempt to enhance the appearance and performance of a vehicle. Using a larger diameter rim while lowering the tire profile, (generally) keeps the overall diameter about the same.

Recommended Inflation Pressure: The recommended inflation pressure for tires – specified in pounds-per-square inch or kilopascals – are set by the vehicle maker and are indicated on each vehicle’s tire placard, certification label or in the owner’s manual. The tire placard is usually located along the driver’s-side door jamb. The recommended inflation pressure differs from the maximum inflation pressure, which is molded into the tire sidewall and should be considered only in extreme cases, usually outlined in the vehicle’s owner’s manual.

Rim: Portion of a wheel to which a tire is mounted.

Rim Diameter: The diameter, in inches, of a wheel measured at the bottom of the flange; always replace a tire with one of the same rim diameter designation.

Rolling Radius: Distance from the center of the wheel (axle) to the road, regardless of condition, load and inflation.

Rolling Resistance: Force needed to keep a tire moving at a constant speed. Low rolling resistance requires the less energy to keep a tire moving, which can translate to greater fuel savings.

Rotation: Moving tires from corner to corner (or front to rear in the case of directional treads) on a vehicle according to a set pattern; this helps to provide even treadwear. Usually recommended at 5,000-mile intervals; often undertaken during oil changes.

Run-Flat Tires: Designed to allow a vehicle to travel with a deflated tire under specific conditions, which generally include speeds no greater than 50 miles. Reduces need to carry spares.

Section Width: The cross-sectional distance between outside sidewalls of inflated tires (LESS load), not including any lettering or design.

Sidewall: The part of a tire between tread and bead.

Sipes: See Micro Grooves.

Size (Tire Size): Measurement to differentiate tires by defining width, rim diameter, aspect ratio and construction type: 225/60HR17, for example, indicates 225 is the tread width (in millimeters), 60 is the aspect ratio, H is the speed rating, R is radial and 17 is the rim diameter (in inches). Other sidewall information may include intended service type, normal section width and load capacity.

Stagger: A vehicle with larger-sized (or wider) wheels and tires on the rear axle with smaller wheels/tires up front. Stagger or staggered fitment is most frequently used on sports cars.

Speed Rating: An indexed alphabetical code that refers to the maximum speed a tire is rated by the tire manufacturer. In order, from lowest speed rated to greatest: a T rated tire is for family sedans and minivans with a 118-mph-top-speed rating, whereas a Q rating is used on winter tires and indicated a top speed rating of 99 mph. Tires for performance cars normally carry Z, W, or Y ratings: A Z-rated tire is rated for speeds above 149 mph with no upper limit specified; W-and Y-rated tires have maximum speed ratings of 168 and 186 mph, respectively.

Stability: The degree to which a tire responds to steering inputs and to external forces.

Squirm: Rubbing motion of tire tread to road surfaces as a loaded tire rotates.

Summer Tires: Designed with materials to perform on wet and dry roads, but are not recommended for ambient temperatures below 40 degrees Fahrenheit.

Tire Balancing: Act of adding external weights to wheel rims to achieve equal weight distribution to improve ride.

Tire Pressure Gauge: A tool used to measure a tire’s air pressure.

Tire Pressure Monitoring System (TPMS): An automated system that remotely monitors air pressure in a vehicle’s tires. It can alert a driver when tire pressures drop below a pre-determined level. Installed in all 1998 and newer model-year cars. It is illegal in the U.S. to disable a vehicle’s TPMS.

Traction: Grip or friction between tires and the road surface under load.

Tread: The part of a tire that contacts a road surface.

Tread Depth: Distance measured from a treads surface to the bottom of a tire’s grooves.

Treadwear Indicators: Molded raised bars at a height of 2/32nds inch found across the bottom of the tread grooves on a tire. As the tread wears to these bars, the tires should be replaced.

Understeer: A vehicle’s handling characteristic where front tires tend to turn less sharply than from where wheels are pointed. Race drivers say understeer is when the front end of a car wants to “push” toward a wall instead of turn; oversteer is when the back of a car wants to slide toward a wall.

Uniform Tire Quality Grade (UTQG): A series of ratings proscribed by the U.S. government and determined by the tire manufacturer. That rates a tire’s traction, temperature resistance and treadwear potential.

Winter/Snow Tires: A winter tire’s tread design and rubber compound are made for cold, snowy and/or icy conditions. Identified by M&S, M+S or M/S markings on the sidewalls. Tires designed for use in severe-snow conditions are further identified with a pictograph of a mountain with a snowflake on the sidewalls and must meet specific snow-performance test requirements.

Zero Offset: When a wheel mounting surface is aligned with a wheel’s centerline.