Tire Glossary

Load Index

A numerical code that specifies the maximum load a tire can carry at the speed indicated by its Speed Rating.

Speed Rating

A letter code that indicates the maximum speed capability of a tire under specified service conditions (e.g., H = 130 mph).

UTQG

Uniform Tire Quality Grading. A government-mandated rating for treadwear, traction, and temperature resistance.

Treadwear

Part of UTQG; a comparative rating based on controlled tests. Higher numbers generally indicate longer tread life.

Traction Rating

Part of UTQG; a measure of a tire’s ability to stop on wet pavement (AA, A, B, C).

Temperature Rating

Part of UTQG; a measure of a tire’s heat resistance (A, B, C).

Offset (ET)

Distance between the wheel mounting surface and the wheel centerline. Positive = closer to the street side; negative = closer to the suspension.

Backspacing

Distance from the wheel mounting surface to the back edge of the wheel. Affects inner clearance to suspension components.

Sidewall Height

The section height of the tire sidewall. Calculated as section width × aspect ratio.

Overall Diameter

Total height of the tire from the ground to the top of the tread. Rim diameter + 2 × sidewall height.

Circumference

Distance around the tire; used to determine revs per mile and speedometer accuracy.

Revs per Mile

How many times the tire rotates over one mile. Changes with tire diameter and affects speedometer readings.

Plus Sizing

Choosing a larger wheel diameter with a lower-profile tire to maintain similar overall diameter.

Minus Sizing

Choosing a smaller wheel diameter with a taller tire to maintain similar overall diameter.

Bead

The part of the tire that sits on the wheel rim. Proper bead seating is critical for safety.

DOT Code

Department of Transportation code on the tire sidewall that includes manufacturing date and plant information.

TPMS

Tire Pressure Monitoring System. Warns drivers when tire pressure is low.