This page describes the formulas used by the calculators presented here. Results compare nominal dimensions; they do not establish manufacturer approval or vehicle-specific physical clearance.
Units
Calculations run internally in millimetres. Inch values convert at exactly 25.4 mm per inch. Rounding happens only for display.
Tire geometry
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| Quantity | Formula |
|---|---|
| Sidewall height | width × aspect ratio / 100 |
| Overall diameter | rim diameter × 25.4 + 2 × sidewall |
| Circumference | π × diameter |
| Revolutions per km | 1,000,000 / circumference (mm) |
These are nominal values from the size markings. A mounted, loaded tire is typically different.
Comparing two tires
Differences are proposed minus current, so a positive value means larger. Ride-height change is half the diameter difference. Percent change is relative to the current diameter.
Speedometer difference
Actual speed equals indicated speed × proposed diameter / current diameter. This assumes the speedometer read correctly on the current tire and that it is driven by wheel rotation. Units are unchanged.
Wheel position
Effective offset is offset minus spacer thickness. Measured from the mounting plane to the nominal bead-seat edges:
- Inner extent = half the nominal width + effective offset.
- Outer extent = half the nominal width − effective offset.
Extents are to bead-seat edges, not to the rim flange or the tire. Real clearance must be measured.
Backspacing
Backspacing = overall width / 2 + offset, where overall width is the measured width including flanges. The calculator does not add a fixed inch to nominal width, because that convention varies; you supply the measurement.
Validation
Non-finite or non-positive dimensions are rejected rather than silently corrected. See testing and accuracy for limits.