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Tyre size comparison calculator

Compare two metric tyre sizes and see how their nominal dimensions affect axle height and estimated road speed.

Compare your tyres

Enter sizes such as 120/70 R17.

First tyre · baseline
mm
%
in
Second tyre · comparison
mm
%
in
Optional speedometer reading
km/h

Use positive values: width up to 1,000 mm, aspect ratio up to 100%, rim up to 50 inches. Speed may be 0–1,000 km/h.

Results update as you type
Estimated speed with second tyre
96.0 km/h

At an indicated 100.0 km/h, with the first tyre as baseline.

Nominal tyre dimensions
Dimension120/70 R17120/60 R17
Sidewall84.0 mm72.0 mm
Diameter599.8 mm575.8 mm
Circumference1884.3 mm1808.9 mm
Diameter difference−24.0 mm (−4.00%)
Circumference difference−75.4 mm (−4.00%)
Approx. radius / axle-height change−12.0 mm

Differences are second tyre minus first; percentages use the first tyre as baseline. Negative values mean a smaller tyre.

How it works

From tyre size to nominal dimensions

In 120/70 R17, 120 is the section width in millimetres, 70 is the sidewall height as a percentage of width, and 17 is the rim diameter in inches.

Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter × 25.4 + 2 × sidewall height. Circumference = π × overall diameter. The approximate axle-height change is half the diameter difference.

Estimated actual speed = indicated speed × second circumference ÷ first circumference. This assumes the speedometer is accurate with the first tyre and that its reading changes in proportion to tyre rotation.

These are nominal estimates, not precise speedometer calibration. Actual rolling circumference depends on the tyre model, load, pressure and wear. Existing speedometer error and the motorcycle’s speed-sensing system also affect the result. Check manufacturer fitment guidance before changing tyre size.