Air Pressure
The measure of force by which the air inside a tire is pressing outward, which is expressed in pounds per square inch (PSI), or kilopascals (kPa), the metric designation for air pressure.
Alignment
When all wheels on the vehicle are adjusted so that they are pointed in the optimum direction relative to the road and each other.
All-season tires
Tires that provide a good balance of traction in rain or snow with good tread life and a comfortable, quiet ride. Check the other types of tires.
Aspect Ratio
The relationship of a tire’s sidewall height to its section width. In a tire size designation, it is 65 in "195/65R15". It is also referred to as the tire’s profile or the series. Read more on the tire designation and the tire aspect ratio.
Aquaplaning
An extremely dangerous situation where water builds up in front of the tires resulting in the tires losing contact with the road surface. At this point, the vehicle is skimming on the water surface and is completely out of control. Also called hydroplaning.
Backspacing
Also known as rearspacing, this is the distance from the mounting pad to the back edge of the rim. Not to be confused with wheel offset.

Wheel Backspace Calculator can help you to calculate the wheel backspace.

Balance/Imbalance
The even distribution of weight on a mounted wheel and tire. To correct an imbalance, a trained mechanic will add weights on the interior or exterior of the wheel.
Bead
The part of the tire that sits on the wheel made of steel wires, reinforced by body ply cords, shaped to hold firmly the tire against the wheel rim.
Bead Chafer
A key component of the tire that is the contact point between the tire and the wheel, designed to withstand forces the wheel puts on the tire during mounting as well as the dynamic forces of driving and braking.
Bolt Circle
The diameter of an imaginary circle drawn through the center of each lug nut hole and then measured from two holes that are directly across from each other. The measurement is used in selecting the proper wheel for replacement.
Bolt Pattern
The arrangement of bolt holes on a wheel. Some wheels have more than one bolt pattern on the same wheel to accommodate multiple fitments.
Breakaway
A term used to describe a loss of traction when negotiating a curve or when accelerating from a standing start. The tires slide against, instead of grip, the road surface.
Carcass
The supporting structure of the tire consisting of plies anchored to the bead on one side and running in a radius to the other side and anchoring to the bead. Also called casing.
Carrying Capacity
At a given air pressure, how much weight each tire is designed to carry. For each tire size, there is a load inflation table to ensure the inflation pressure used is sufficient for the vehicle axle load.
Cold Inflation Pressure
The amount of air pressure in a tire, before a tire has built up heat from driving.
Contact Patch
The area in which the tire is in contact with the road surface. Also called footprint.
DOT Markings
A designation molded into the sidewall of a tire consisted of the DOT mark and 10-digit code. Provides information such as the week and year the tire was produced, plus the manufacturer, plant, tire line, and size. DOT is an acronym that stands for Department of Transportation.
Grooves
Circumferential channels between the tread ribs of a tire.
Load-carrying capacity
Indicates how much weight a tire is certified to carry at maximum inflation pressure.
Load index
An assigned number ranging from 0 to 279 that corresponds to the load-carrying capacity of a tire. See this load index table.
Low Profile
A term describing a tire with a low relative aspect ratio or series classification (short sidewall, wide tread).
Maximum inflation pressure
The maximum air pressure to which a cold tire may be inflated; can be found molded onto the sidewall.
Metric tire size system
One system used to describe a tire’s size. It is the standard system of the ETRTO (European Tire and Rim Technical Organization).
Negative offset
When the wheel mounting face is closer to the brake side of the wheel, moving the tire and wheel assembly out of the fender well.
Nominal Rim Diameter
The diameter of a tire rim, given in nearest whole numbers (e.g. 15 in.).
Offset
The offset of the rim is what locates the tire/wheel assembly in relation to the suspension. A wheel with zero offset has a mounting face that directly aligns to the wheel’s centerline.
A positive offset means the wheel’s mounting surface is positioned in front of the centerline of the wheel/tire assembly, bringing the tire closer to the suspension. Conversely, a negative offset means that a wheel is behind the centerline of the wheel/tire assembly, causing the tire moves further from the suspension, to stick out away from the vehicle.

Wheel Offset Calculator can help you to calculate the wheel offset.

Original Equipment Manufacturer - OEM
Tires selected by a vehicle manufacturer that best match tire performance to vehicle performance characteristics. Also known as the Stock tire size.
Overall diameter
The diameter of the inflated tire, without any load.
Overall width
The distance between the outside of the two sidewalls, including lettering and designs.
Overinflation
Too much air in the tire, resulting in premature wear in the center of the tread.
Plus-sizing
An option allowing drivers to customize their vehicles by mounting low-profile tires on wider rims (one or two inches greater in diameter), usually enhancing vehicle appearance, handling, and performance. It is recommended that you keep the overall tire diameter within the certain range of the OEM tire size to prevent problems with spedometer, transmission, gas mileage, and braking. Read more about plus-sizing and try our plus sizing calculator.
Ply
A rubber-coated layer of fabric containing cords that run parallel to each other and make up the structure of a tire. Layers of this material are called plies, and they extend from bead to bead, between the inner liner, and belts or tread. Plies are usually reinforced with either textile or steel cords.
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 is P205/70R14 93S.
Pneumatic tire
A tire designed to be filled with air.
Positive offset
The mounting face of a wheel is toward the wheel’s street side, moving the tire and wheel assembly in toward the vehicle.
PSI
Abbreviation for pounds per square inch, which is the automotive industry’s measurement of the pressure in a tire.
Radial ply tire
A type of tire with plies arranged so cords in the body run at 90-degree angles to the center line of the tread.
Revolutions Per Mile (RPM)
Also called rpm. Measured number of revolutions for a tire traveling one mile. This can vary with speed, load, and inflation pressure. Use our Tire Size Calculator to find out the revolutions per mile for the tire.
Ribs
A pattern of tread features aligned around the circumference of a tire. There are usually multiple ribs across the tread area of a tire.
Rim
That portion of a wheel to which a tire is mounted.
Rim diameter
The diameter of the rim bead seats supporting the tire.
Rim width
The measurement between the flanges of a rim. You can use our Wheel/Rim Size Calculator to find out which rim is appropriate for the selected tire size.
Rim drop
Also called drop center, a change (drop) in the rim profile between the rim flanges in which the bead area of a tire is placed during the mounting process. This allows the tire to be mounted on the rim.
Rim flange
Surface of the rim of the wheel that contacts the side of the tire bead.
Rolling circumference
The linear distance traveled by a tire in one revolution (its circumference). This can vary with load and inflation. Rolling circumference can be calculated as follows: 63,360 divided by revolutions per mile = rolling circumference in inches. Try out our Tire Size Calculator which will give you info about tire diameter, circumference, etc.
Rolling resistance
The force required to keep a tire moving at a uniform speed. The lower the rolling resistance, the less energy needed to keep a tire moving.
Rotation
The changing of tires from front to rear or from side to side on a vehicle according to a set pattern; provides even treadwear. Rotating your tires on a regular basis (every 6,000-8,000 miles) is a simple way to add miles to their life. See your tire warranty for more information on recommended rotation. Find out how to make your tires last longer.
Section height
The height of a tire, measured from its rim to its outer tread. Our calculator can give you the information about all tire dimensions.
Section width
The distance between the outside of a tire’s sidewalls, not including any lettering or designs.
Self-aligning torque
When the tire is cornering, torque created at the road contact patch acts at a point somewhat to the rear of the actual wheel center due to pneumatic trail. This has the same effect as positive caster and tends to force the wheel back to the straight-ahead position.
Series
Tires with the same aspect ratio, or relationship of section height to section width.
Sidewall
That portion of a tire between the tread and the bead. Protects the tire against impacts with curbs, etc. This is also where the sidewall markings can be found which tell you important information regarding the tire.
Speed rating
An alphabetical code (A-Z) assigned to a tire indicating the range of speeds at which the tire can carry a load under specified service conditions. Check this speed index table.
Standard load
The amount of weight a given size tire can carry at a recommended air pressure.
Symmetrical Tread Design
Uniform tread pattern on both sides of the tread for better performance in specific conditions and on specific roads.
Tire
Also called pneumatic tire, a precisely engineered assembly of rubber, chemicals, fabric, and metal, designed to provide traction, cushion road shock and carry a load under varying conditions.
Tire Size
The combination of tire width, construction type, aspect ratio, and rim size used in differentiating tires.
Tire Designation
An alphanumeric code molded into the sidewall of the tire that describes the tire’s size, including width, aspect ratio, rim diameter, load index, and speed rating. Most designations use the P-Metric system.
Tire placard
A metal or paper tag permanently affixed to a vehicle, which indicates the appropriate tire size and inflation pressures for the vehicle. The placard can ordinarily be found on either the driver’s doorpost, the glove box lid, or the fuel-filler door.
Tire pressure gauge
Tool used to properly measure the air pressure in a tire.
Traction
The friction between the tires and the road surface; the amount of grip provided.
Tread
That portion of a tire that comes into contact with the road. It is distinguished by the design of its ribs and grooves. Provides traction in a variety of conditions, withstands high forces, and resists wear, abrasion, and heat.
Tread Depth
The depth of usable tread rubber, the distance from the top of the tread to the grooves in a tire. The measurement is taken at the centerline of a tire it's measured in 32nds of an inch. If a tire comes new with 10/32nds of rubber, you have 8/32nds of usable rubber. Tires must be replaced when the wear bars are visible at 2/32nds.
Tread life
The life of a tire before it is pulled from service; mileage.
Treadwear indicator
Narrow bands, sometimes called wear bars, that appear across the tread of the tire when only 2/32 inch of tread remains.
Tread pattern
The arrangement of grooves, blocks, sipes, and channels on the tread of a tire that provide a varying degrees of effects on traction, tread life, and other tire factors.
Tread width
The width of a tire’s tread.
Underinflation
Operating a tire without sufficient air pressure to support the weight of the vehicle with occupants and additional load; could cause failure of the tire when heat is generated inside the tire to the point of degeneration of components.
Valve
A device that lets air in or out of a tire. It is fitted with a valve cap to keep out dirt and moisture, plus a valve core to prevent air from escaping.
Wheel weights
Small weights attached or secured to the wheel to balance the tire and wheel assembly.
Zero offset
When the mounting face of the wheel directly aligns with the wheel’s centerline.