Tire and Wheel Glossary

Tires and wheels are things that seem fairly simple -- but once you start peeling back the layers you realize they're actually quite complicated. In an effort to demystify the world of tires and wheels, with its confusing terminology, technology, and "tirespeak," we present the TireBuyer Tire and Wheel Glossary.

A

Alignment

Adjusting the vehicle's wheels, steering, and suspension components to manufacturer's specifications. When wheels are in alignment, they are in the optimal position relative to the road and each other. Impacts like driving over a curb or hitting a pothole can cause misalignment.

All-season tires

Tires that are built to provide good performance all year round. The Rubber Manufacturer's Association defines the specifications for all-season tires. All-season tires have a MS, M/S, or M&S mark on the sidewall, meaning that they are suitable for use in mud and snow.

Aspect ratio

The relationship of the sidewall's height to its width. For example, if the sidewall's height is 45% of its width, the aspect ratio is 45. In the tire size 225/45R17, the aspect ratio is 45.

Asymmetric or asymmetrical

A tire design where the tread pattern on one side of the tire is different than the tread pattern on the other side.

B

Balancing

Adjusting tires and wheels so that they spin with evenly distributed weight. Mechanics use weights on the wheel to correct any imbalances.

Bead

A round hoop of steel wire and steel reinforcements at the inside of the tire, where it connects to the wheel.

Belt

A layer of cords wrapped in rubber, located between the tire tread and the body plies. The cords are usually made of steel, but other materials include fiberglass, nylon, rayon, and polyester.

C

Carcass

The supporting structure of a tire, underneath the tread. Made up of plies anchored to the beads on each side.

Contact patch

The part of the tire that comes into contact with the road. Also known as the footprint.

D

D.O.T. number

A 10 - 12 character code branded on the sidewall of tires sold in the United States. The code signifies that the tire meets the Department of Transportation's standards and includes information like tire size, type, manufacturer, manufacturing location, and date of manufacture (week and year).

Directional tread

Tires with directional tread are made to roll in one direction only. As a result, directional tires are used on one side of the vehicle; when they're rotated they move from front to back.

F

Flat spot

Irregular wear in an isolated area of a tire's tread.

H

Hydroplaning

A condition where water on the road can't be moved out from beneath the tires, causing the vehicle to stop responding to the steering wheel and ride on a layer of water instead of the pavement.

L

Load carrying capacity

The maximum amount of weight a tire can carry under ideal conditions.

Load index

An assigned number from 0 to 279 that denotes the amount of weight a tire can safely carry.

Low-profile tire

A tire with a short sidewall (and a lower aspect ratio). Generally high-performance tires, low-profile tires offer improved handling, but the ride can be rougher since there's less cushioning between the wheel and the road surface.

M

M+S, M/S, M&S

Mud and snow - a designation appearing on the sidewall of all-season tires, denoting that the tire meets the Rubber Manufacturer's Association's definition of a mud and snow tire.

Mounting

The act of installing a tire on a wheel. New tires should always be professionally mounted and balanced.

N

Negative camber

Alignment style where the tops of the tires are angled in towards the center of the vehicle. Generally used on race cars for improved grip while cornering.

Negative offset

When the wheel mounting surface is toward the back, or brake side of the wheel. In a wheel with negative offset, the tire and wheel are moved out of the wheel well.

O

Offset

The distance from the wheel mounting surface to the centerline of the wheel.

Original equipment (OE)

The tires included with a new vehicle when it is purchased. OE tires are specifically chosen by the vehicle manufacturer to make the most of the vehicle's performance characteristics.

P

Penny test

A simple test to check tire for proper tread depth, using a penny.

Plus sizing

Enhancing the appearance and performance of a vehicle by adding larger diameter wheels and lower profile tires. Using a larger diameter rim while lowering the profile of the tire keeps the overall diameter about the same size.

Ply/plies

Rubber-coated layer(s) of fabric containing cords that make up the internal structure of a tire. Plies run from bead to bead, in between the inner liner and the tire tread.

Positive camber

Alignment style where the tops of the tires are angled out from the center of the vehicle.

Positive offset

When the wheel mounting surface is toward the front, or street side of the wheel. In a wheel with positive offset, the tire and wheel are moved in toward the vehicle.

R

Rolling resistance

The force needed to keep a tire rolling at a uniform speed. The lower the rolling resistance, the less energy is required.

Rotation

Moving tires from front to rear, side to side, or in another set pattern in order to promote even tread wear and longer life.

Run flat

Tires built with self-supporting sidewalls that let you keep driving after a puncture - at reduced speed and for a limited distance.

S

Shoulder

Where the tire's sidewall meets the tread.

Sidewall

The side of the tire, between the tread surface and the bead. The location of information about the tire including size, date of manufacture, construction, and more.

Sipes

Small slits within the tire's tread that increase traction in wet and snowy conditions. Sipes work by opening as the tire rolls onto the road, gripping the road surface and keeping more rubber on the road.

Speed rating

An alphanumeric code denoting the top speed at which a particular tire can travel safely. For example, a T-rated tire is made for family sedans and vans, and has a top speed of 118 mph. A Y-rated tire is meant for exotic sports cars, so it has a much higher top speed - 186 mph.

Staggering/staggered fitment

A vehicle that has larger-sized (or wider) wheels on the back axles and smaller wheels on the front. Staggered fitment is most frequently seen on sports cars.

Steel belted radial

A radial tire made with steel belts (rather than fabric belts).

Studs

Metal or plastic pins that can be inserted into the tread of a studdable tire to increase traction in severe ice and snow.

T

Tire disposal/recycling fee

A state-mandated fee collected by tire sellers in many states to fund scrap tire cleanup and recycling programs. The fee varies by state, from less than $1.00 per tire to $4.00 per tire.

Tire size

A collection of numbers and letters that tells you the tire width, sidewall height (aspect ratio) and wheel diameter, among other information. For example, the tire size 225/45R17 tells you that the tire has a width of 225 millimeters, an aspect ratio of 45%, and a wheel diameter of 17 inches. Tire size can be found on the sidewall of a tire.

TPMS

TPMS stands for Tire Pressure Monitoring System. It’s a safety system built into your vehicle (or retrofitted) that monitors your tire pressure, and alerts you when the pressure in one or more tires falls to an unacceptable level.

Tread

The outer layer of a tire; the part that comes into contact with the road.

Tread depth

Measurement from the bottom of a tire's tread grooves to the tread surface, measured in 32nds of an inch. New car tires generally have a depth of 10/32" or 11/32" (truck tires and winter tires are usually deeper); tires are considered worn out at 2/32" (or in some cases, earlier).

Tread life

The expected service length of a tire before it wears out, measured in miles.

Tread wear indicator/tread wear bar

Bands built into a tire's tread that alert the owner when the tire has reached 2/32nds of tread depth and needs to be replaced.

U

UTQG

Uniform Tire Quality Grade. A government-sponsored tire information program that provides ratings for tires' traction, temperature resistance, and treadwear.

V

Valve assembly

Device that lets air in or out of the tire. Includes the valve core which keeps air from escaping, the valve stem (tube), and the valve cap/cover to keep out dirt and moisture.

Vehicle placard

A sticker that contains a variety of important information about your vehicle's tires, including the vehicle manufacturer's recommendations for tire size, tire pressure, load carrying capacity, and speed rating. Usually located on the driver's side door jamb.

W

Wheel weight/wheel balance

Small metal weight attached to the inside or outside of the wheel to balance the tire and wheel assembly.

Winter tires/snow tires

Tires built with a special tread compound and tread pattern to optimize traction in winter weather conditions. Winter tires are branded with the "mountain/snowflake" symbol on the sidewall.

Z

Zero offset

When the wheel mounting surface is aligned with the wheel's centerline.

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