Wheel & Tire Glossary
A comprehensive reference of wheel and tire terminology for automotive professionals. Use this glossary to understand specifications, communicate with customers, and make informed fitment decisions.
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A
- Aspect Ratio
- The ratio of a tire's sidewall height to its section width, expressed as a percentage. In tire size 225/55R17, the aspect ratio is 55, meaning the sidewall height is 55% of the 225mm width.
B
- Backspacing
- The distance from the wheel's mounting surface to the back edge of the wheel. Combined with wheel width, it determines offset.
- Bead
- The inner edge of a tire that seats against the wheel rim. The bead is reinforced with steel wire to maintain a secure seal with the wheel.
- Bias-Ply Tire
- An older tire construction where internal plies cross diagonally from bead to bead. Less common today, mostly found on trailers and vintage vehicles. Compare with Radial Tire.
- Bolt Pattern
- The arrangement of lug holes on a wheel, defined by the number of bolts and the diameter of the circle they form. Also known as PCD (Pitch Circle Diameter). Example: 5x114.3 means 5 bolts on a 114.3mm diameter circle.
C
- Camber
- The vertical tilt of the wheel when viewed from the front of the vehicle. Negative camber tilts the top of the wheel inward; positive camber tilts it outward. Affects tire wear and handling.
- Centerbore
- The hole in the center of a wheel that fits over the vehicle's hub. A hub-centric wheel has a centerbore that matches the hub diameter exactly, while lug-centric wheels rely on lug nuts for centering.
- Contact Patch
- The area of the tire that is in contact with the road surface at any given moment. A larger contact patch generally provides more grip.
D
- Directional Tire
- A tire with a tread pattern designed to rotate in only one direction. Marked with an arrow on the sidewall indicating proper rotation. Cannot be rotated side-to-side.
- DOT Code
- A code on the tire sidewall required by the Department of Transportation. The last four digits indicate the week and year of manufacture (e.g., 2521 = 25th week of 2021).
F
- Flotation Tire
- A tire sizing system common on trucks and off-road vehicles. Example: 31x10.50R15 indicates a 31-inch overall diameter, 10.5-inch section width, radial construction, and 15-inch wheel diameter. See also Metric Sizing.
H
- Hub Ring
- A plastic or metal ring used to convert a lug-centric wheel to hub-centric fitment. Fills the gap between the wheel's centerbore and the vehicle's hub.
- Hub-Centric
- A wheel design where the centerbore precisely matches the vehicle's hub diameter, ensuring the wheel is centered by the hub rather than the lug nuts. Provides better balance and reduces vibration. Compare with Lug-Centric.
L
- Load Index
- A numerical code on a tire indicating its maximum load capacity when properly inflated. Each number corresponds to a specific weight capacity (e.g., load index 91 = 1,356 lbs per tire). See also Load Range.
- Load Range
- A letter designation (B, C, D, E, F, etc.) indicating a tire's ply rating and maximum inflation pressure. Higher load ranges support heavier loads. Common on light truck tires.
- LT (Light Truck)
- A tire type designation for tires designed for light trucks, larger SUVs, and vans. LT tires have stronger sidewalls and higher load capacities than P-metric tires.
- Lug-Centric
- A wheel mounting style where the wheel is centered by the lug nuts rather than the centerbore. Hub rings are often used to convert lug-centric to hub-centric mounting.
M
- M+S (Mud and Snow)
- A tire marking indicating the tread design meets certain traction requirements for mud and snow conditions. Not as stringent as the 3PMSF (Three-Peak Mountain Snowflake) winter rating.
- Metric Sizing
- The standard tire sizing system used worldwide. Example: P225/55R17 indicates Passenger tire, 225mm section width, 55% aspect ratio, Radial construction, 17-inch wheel diameter. See also Flotation Tire.
- Minus Sizing
- Decreasing wheel diameter while increasing tire sidewall height to maintain approximately the same overall diameter. Often used for winter tire setups to improve ride comfort and protection.
N
- Negative Offset
- When the wheel mounting surface is closer to the back of the wheel than the centerline. Pushes the wheel outward from the vehicle, creating a wider stance. See also Offset and Positive Offset.
O
- OE (Original Equipment)
- The tires, wheels, or specifications that came installed on a vehicle from the factory. OE specifications are the baseline for fitment recommendations.
- Offset
- The distance in millimeters from the wheel's centerline to its mounting surface. Positive offset means the mounting surface is toward the front (street side) of the wheel; negative offset means it's toward the back. See also Backspacing.
- Overall Diameter
- The total height of the mounted tire and wheel assembly, measured from ground to top of tire. Critical for speedometer accuracy and preventing rubbing issues.
P
- P-Metric
- A tire type designation for passenger vehicle tires. The P prefix indicates the tire is designed for cars, minivans, and light-duty SUVs. Compare with LT (Light Truck).
- PCD (Pitch Circle Diameter)
- See Bolt Pattern. The diameter of the circle formed by the center of the wheel's lug holes.
- Plus Sizing
- Increasing wheel diameter while decreasing tire sidewall height to maintain approximately the same overall diameter. Improves handling and appearance while maintaining speedometer accuracy.
- Ply Rating
- A historical measure of tire strength based on the number of cotton ply layers. Modern tires use stronger materials but retain ply rating terminology to indicate load capacity. See also Load Range.
- Positive Offset
- When the wheel mounting surface is closer to the front (street side) of the wheel than the centerline. Pulls the wheel inward toward the vehicle. See also Offset and Negative Offset.
R
- Radial Tire
- A tire construction where the internal plies run perpendicular (radially) to the direction of travel. Indicated by 'R' in the tire size. Offers better fuel economy, tread life, and ride comfort than bias-ply tires.
- Revolutions Per Mile
- The number of complete rotations a tire makes to travel one mile. Changes with tire overall diameter. Affects speedometer accuracy when changing tire sizes.
- Rolling Resistance
- The energy required to keep a tire rolling. Lower rolling resistance improves fuel economy but may reduce grip.
- Run-Flat Tire
- A tire designed to continue functioning after a puncture or loss of air pressure, typically for 50 miles at reduced speed. Uses reinforced sidewalls or internal support rings.
S
- Section Width
- The measurement of a tire's width from sidewall to sidewall, not including any lettering or protective ribs. In tire size 225/55R17, the section width is 225 millimeters. Used to calculate aspect ratio.
- Sidewall
- The portion of the tire between the tread and the bead. Contains tire information markings and provides flexibility for ride comfort.
- Speed Rating
- A letter code indicating the maximum sustained speed a tire can safely handle. Common ratings: S (112 mph), T (118 mph), H (130 mph), V (149 mph), W (168 mph), Y (186 mph).
- ST (Special Trailer)
- A tire type designation for tires designed specifically for trailers. ST tires have stiffer sidewalls to handle trailer loads and are not rated for steering or driven axles.
- Staggered Fitment
- Using different width wheels and tires on the front and rear of a vehicle. Common on performance and luxury vehicles for improved handling and appearance.
- Symmetric Tread
- A tire tread pattern that is the same on both halves of the tire. Can be rotated to any position on the vehicle. Compare with Directional Tire.
T
- Tire Pressure Monitoring System (TPMS)
- An electronic system that monitors tire air pressure and alerts the driver when pressure is significantly low. Required on all U.S. vehicles since 2007.
- Torque Specification
- The recommended tightening force for lug nuts, measured in foot-pounds (ft-lbs) or Newton-meters (Nm). Proper torque ensures safe wheel attachment and prevents damage.
- Tread Depth
- The vertical measurement from the top of the tread to the bottom of the deepest grooves. New tires typically have 10/32" to 11/32" depth; 2/32" is the legal minimum in most states.
- Treadwear Rating
- Part of the UTQG system, a number indicating relative tread life. A tire rated 400 should last twice as long as one rated 200 under controlled conditions.
U
- UTQG (Uniform Tire Quality Grading)
- A rating system required by DOT showing Treadwear, Traction (AA, A, B, C), and Temperature (A, B, C) grades. Helps consumers compare tire performance.
W
- Wheel Diameter
- The diameter of the wheel measured at the bead seat where the tire mounts, not including the tire. Specified in inches (e.g., 17" wheel).
- Wheel Spacer
- A device installed between the wheel and hub to push the wheel outward from the vehicle. Changes the effective offset and can widen the vehicle's track.
- Wheel Width
- The measurement between the inner flanges of the wheel where the tire beads seat. Specified in inches (e.g., 8" wide wheel). Determines which tire section widths are compatible.
X
- XL (Extra Load)
- A tire designation indicating the tire has a reinforced construction to carry heavier loads at higher inflation pressures. Common on performance and luxury vehicles.
Z
- Zero Offset
- When the wheel mounting surface is exactly at the wheel's centerline. The wheel is neither pushed in nor out from its centered position. See also Offset.
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