Speedometer Error From Tyre Size Calculator

Work out speedometer error from tyre size instantly with clear inputs, formula shown and shareable results.

About Speedometer Error From Tyre Size Calculator

The speedometer counts wheel revolutions, so a larger tyre travels further per revolution and the true speed exceeds the indication. Tread wear works the other way, reducing diameter by twice the wear depth and making the reading slightly optimistic again.

Formula

Speedometer error

actual speed = indicated x new diameter / original diameter

Worked example

With the default values below, Speedometer Error From Tyre Size Calculator returns Actual speed of 100.95. Change any input above and every figure here is recalculated in your browser.

Inputs used

Original tyre diameter: 632 mm
New tyre diameter: 646 mm
Indicated speed: 100 km/h
Tread worn since new: 4 mm

Results

Actual speed: 100.95 km/h
Speedometer error: 0.949% %
Odometer error per 1000 km: 9.49 km
Diameter allowing for wear: 638.00 mm

Step by step

1Values used
2Speedometer error
3Actual speed= 100.95 km/h
4Speedometer error= 0.949 %
5Odometer error per 1000 km= 9.49 km
6Diameter allowing for wear= 638.00 mm

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Does tread wear affect the reading?

Yes, but only slightly. Losing 4 mm of tread reduces diameter by 8 mm, about 1.3% on a 632 mm tyre.

Is it legal to fit a different size?

Generally only sizes approved for the vehicle. A significantly different rolling radius affects braking systems, stability control and the accuracy of legally required instruments.