Tyre Speed Difference Calculator

How to use Tyre Speed Difference Calculator

1. Enter your figures in the units shown beside each field. Keep monetary inputs in the same currency; money results are currently displayed in USD.
2. Select the unit standard where the form offers a choice, then choose Calculate.
3. Read the breakdown and compare the worked example below. Change one assumption to see its effect.

Formula used

Diameter change % = (new diameter/old diameter − 1) × 100. Estimated actual speed = indicated speed × new diameter/old diameter.

Worked example

Changing 225/45R17 (634.3 mm) to 235/40R18 (645.2 mm) increases nominal diameter by 1.72%. An indicated 60 corresponds to about 61.03 in the same speed unit.

What the result means

The speed calculation assumes the old size was the calibration reference. Actual speedometer bias and loaded tyre dimensions are not modelled.

Check your result

Results retain calculation precision internally and round for display. Keep the input values with the result so you can reproduce it. Use Reset to restore the example inputs, or Copy result to copy the displayed answer.

If the result does not agree with your own calculation, report the page, inputs and expected answer. See our methodology and reference sources for the standards used.

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