Tire and wheel glossary

The nominal maximum tire width from sidewall to sidewall, measured in millimetres on a specified measuring rim. It is not tread width.

The sidewall height expressed as a percentage of nominal section width. In 225/45R17, 45 means 45% of 225 mm.

The wheel bead-seat diameter, normally stated in inches. The tire and wheel diameter designations must match exactly.

For a metric tire, the calculated radial height from bead seat to tread: section width multiplied by aspect ratio and divided by 100.

The nominal tire diameter: rim diameter plus two sidewalls. Actual loaded and measured diameter can differ from this geometric value.

The nominal distance covered in one revolution, calculated as overall diameter multiplied by pi. Load, pressure, wear, and construction affect the real value.

The estimated number of tire turns per mile or kilometre, calculated by dividing that distance by nominal circumference.

The percentage change from the original overall diameter to the alternative diameter. Positive values mean the alternative is taller.

The estimated indicated-versus-actual speed difference caused by a change in rolling circumference. It excludes calibration and tire deformation effects.

Half of the overall-diameter difference. It estimates the change in axle height before suspension, load, pressure, or tire deflection effects.

A documented threshold for nominal diameter change used as an initial screen. Passing it does not prove clearance, ratings, legality, or manufacturer approval.

The internal width between the wheel bead seats, usually in inches. It is narrower than the wheel’s outside lip-to-lip width.

The distance from the wheel centreline to its hub-mounting face. More positive offset generally moves the wheel further inward.

The distance from the hub-mounting face to the wheel’s inner edge. It depends on wheel width, offset, and the outer-lip allowance.

The number of wheel fasteners and the diameter of the circle through their centres, such as 5x112.

The wheel’s central opening that fits over the vehicle hub. Diameter and hub-centering requirements must be verified for the vehicle.

A component fitted between hub and wheel. Its thickness moves the wheel outward and reduces effective positive offset by the same amount.

A numerical code for the maximum load one tire can carry under the rating standard’s specified conditions. Match vehicle requirements.

A letter code tied to a standardized maximum test speed under stated load and pressure conditions. It is not a recommended driving speed.

Using a larger-diameter wheel with a lower-profile tire while keeping nominal overall tire diameter close to the original setup.