Transparent formulas, defined assumptions
Fitment Calculator uses deterministic wheel-and-tire geometry. Calculations retain full precision internally and round only for display. This page documents the core equations so results can be checked independently.
Tire geometry
- Sidewall height: section width × aspect ratio ÷ 100.
- Overall diameter: rim diameter × 25.4 + 2 × sidewall height.
- Circumference: π × overall diameter.
- Ground-clearance change: half of the overall-diameter difference.
Wheel position and spacers
Nominal wheel-edge positions are calculated from half the nominal bead-seat width and signed offset. Spacer thickness reduces effective positive offset by the spacer thickness. Because nominal wheel width excludes the outer flanges, measured overall width is preferred for physical backspacing.
Speed estimate
Estimated actual speed = indicated speed × new theoretical diameter ÷ original theoretical diameter. This is a dimensional estimate, not an electronic recalibration or legal speed measurement.
What the formulas do not model
Two-dimensional geometry cannot verify brake caliper shape, suspension motion, body clearance, bolt pattern, centre bore, load ratings, tire approval, manufacturer tolerances or local legality. Physical verification remains necessary.