Photo by Josh Kenzer

If you’re interested in tires, the following glossary goes over some tire basics. You’ll know a lot of it already, probably. But there may be a few terms or tidbits that are new to you as well.

Properly maintaining your tires can greatly improve the performance and handling of your vehicle, will give you a smoother ride, better gas mileage, and will save you money over the long haul.

The Tire Glossary

Air Pressure

The amount of air inside the tire, expressed as pounds per square inch. Air pressure should be measured when the tire is cold—meaning before it has been driven on.

All-Season Tires

Tires designed and built to provide reliable traction in all common road conditions, including dry, wet, snow, et cetera. The versatility of all-season tires does lend itself to limited performance in extreme conditions, and certainly compared to purpose-built tires for specific performance categories.

Alphanumeric

A term for describing the size of a tire (215/65R15, for example) using both letters and numbers.

Aspect Ratio

Describes the tire’s height-to-width proportion. Using the previous example, (215/65R15), the number 65 represents the aspect ratio. It means the tire’s sidewall height is 65 percent of its section width.

Balance

Achieved when all four wheels and tires spin with weight distributed evenly. Not balancing wheels can result in tire vibration. A wheel balancer is used to place weights to compensate for static and dynamic imbalances found in all wheel-and-tire assemblies.

Bead

The length of steel wire wrapped and reinforced by steel cords that are built into the tire diameter and run in a complete circle around the tire’s outer diameter to make it strong.

Bias Ply Tire

A bias ply tire is a tire built with criss-crossing plies to give the tire strength. The crossing plies generate heat when in use and ultimately limit the tire’s performance and wear characteristics.

Carcass/Casing

The body of the tire beneath the tread and sidewalls.

Contact Patch

The portion of tread that touches the road while rotating.

Cord

The strands of material that form the tire layers. Cords are often made from steel, fiberglass, polyester, nylon, or rayon.

Footprint

The footprint is the same as the contact patch. It is the portion of tread that touches the road.

Groove

The space between two adjacent tread ribs. Sometimes called “tread grooves.”

Heat Cycling

A way to break-in competition tires before using them in racing conditions. Heat cycling heats tires gradually in a controlled manner. It is designed to give drivers more traction and longer tread life.

Highway Tires

Sometimes called summer tires, highway tires are purpose-built for dry and occasionally wet driving conditions, but not for use on ice or snow.

Hydroplaning

The result of tires losing contact with the road in wet driving conditions, like sliding on ice.

Innerliner

The inside layer of a tubeless tire which prevents air from permeating through the tire wall; a replacement for the innertube.

Load Index

The load carrying capacity of a particular tire, designated by a number ranging from 0-279.

Maximum Inflation Pressure

Indicates the maximum air pressure to which a cold tire can be inflated. The maximum pressure is typically listed on the tire’s sidewall.

Overall Diameter

The diameter of an inflated tire minus any load.

Overall Width

The distance between the tire’s outer sidewalls, including lettering.

P-Metric

Introduced by American tire manufacturers in 1977, P-metric is a uniform designation of tire sizes using metric measurements. A typical example of a P-metric series tire size is P215/70R-15.

Placard

A small label typically found on the edge of the driver’s side door or inside the vehicle glove compartment with information including the vehicle manufacturer’s recommended tire inflation pressure, Gross Vehicle Weight, seating capacity, et cetera.

Plus-Sizing

The act of customizing your vehicle appearance by mounting lower-profile tires on larger-diameter wheels. Using a lower-profile tire with a larger wheel proportionally, allows the overall diameter to remain about the same.

Ply

A rubber-coated layer of cords that run throughout tires. They exist between the inner liner and tread belts. They extend from bead to bead.

Ply Rating

A way to express the load-carrying capacity of tires in terms of the number of plies. However, the number doesn’t necessarily correspond with the specific number of plies. For example, a 6-ply rating is expressed as Load Range C. And it doesn’t have six individual plies. It has one or two plies equivalent to the strength of six plies. Tires run cooler and last longer as a result of this design.

PSI

Otherwise known as Pounds Per Square Inch. This is the standard unit of measurement for your tire air pressure.

Ride Matching

A method of balancing wheel-and-tire assemblies using simulated road tests. Ride matching will give you the optimum weight distribution and prevent tire vibration caused by a series of small imbalances.

Rim/Wheel Width

The distance between the two opposite inside edges of a rim/wheel.

Rolling Resistance

The amount of force required to keep tires moving at a constant speed. As rolling resistance lessens, so does the amount of energy needed to keep it rotating.

Rotation

The act of transferring tires from front to back or side to side on your vehicle so that all of your tires wear at the same, uniform rate. It is generally recommended that you rotate tires every 5,000-6,000 miles.

Section Height

The height of a tire as measured from the outer tread to the wheel edge.

Section Width

The distance between your tires’ outside sidewalls, not including any raised lettering.

Shimmy

The result of wobbling wheels from side to side when the vehicle is in motion. Shimmying is caused by misaligned or bent wheels, or imbalanced tires.

Shoulder

The spot on your tire where the tread meets the sidewall. Some tires feature shoulder blocks for improved traction and performance in certain applications.

Sidewall

The part of your tire between the bead and the tread.

Size

The size of a tire is defined by its height, width, wheel diameter, aspect ratio, and construction.

Snow Tire

Sometimes called winter tires, snow tires are built with compounds and with tread designs that provide improved traction in snowy driving conditions. All-season tires also incorporate some of the same design features.

Speed Rating

Tire speed ratings are determined via laboratory test that simulate performance at various speeds. The speed ratings of tires are based on U.S. government standards for accelerating to, and sustaining, certain speeds. Tires with higher speed ratings will generally give you better handling.

Tire Pressure Monitoring System (TPMS)

An automated system of computer sensors that monitor the air pressure in your vehicle’s tires. TPMS sensors are typically triggered when one or more of the tires drops 25 percent or more below the recommended tire pressure.

Tread

Distinguished by the varying design of ribs and grooves, the tire tread is the part of the tire that comes in contact with the road or driving surface.

Tread Depth

The distance between the top of the tire and the base of the tread groove nearest the tire’s centerline. Most states consider a tire to be worn out when its tread depth reaches 2/32 inch.

Treadwear Indicator

Narrow bands—otherwise known as wear bars—that appear on the tread when the tread depth is worn to 2/32 inch.

Tread Width

The width of your tire’s tread.

Valve

The device that allows air in and out of your tires. Tire valves prevent air from escaping your tires, and valve caps exist to prevent dirt and moisture from building up in your tire valves.