Does fitting bigger tyres make my speedo read slow?

Yes. Bigger tyres cover more ground per revolution, so your speedo shows less than your actual speed. For example, a 3% increase in diameter means when your speedo shows 70, you are actually doing about 72.1 mph.

Is it legal to have speedometer error in the UK?

Your speedometer must never show less than your actual speed, but it can show more. This means fitting larger tyres (which make the speedo read slow) is technically an issue, while smaller tyres (speedo reads fast) is acceptable. UK MOT allows up to 10% over-reading.

Can I recalibrate my speedometer?

Some modern cars allow software recalibration through the dealer. Aftermarket GPS speedometers can also be fitted. For older cars, you can use a GPS app on your phone to check true speed and mentally adjust.

Will changing tyre size affect my odometer?

Yes. Your odometer uses the same wheel speed sensor as your speedometer. If your tyres are 3% larger, your odometer will under-read by 3%, meaning you are actually covering more miles than shown. This also affects service intervals and resale mileage.

What is the maximum speedometer error allowed in a UK MOT?

A speedometer must not show less than the actual speed (it must never under-read). It can over-read by up to 10% plus 6.25 mph at the speed tested. Fitting larger tyres that cause under-reading could result in an MOT failure.

CarCalcs

Speedometer Correction Calculator

Find out how changing tyre size affects your speedometer reading.

Original Tyre

Width

Total width of the area or object

Profile

Tyre aspect ratio as a percentage of width

Rim

Wheel rim diameter in inches

New Tyre

Width

Total width of the area or object

Profile

Tyre aspect ratio as a percentage of width

Rim

Wheel rim diameter in inches

How We Calculate This

The speedometer error is proportional to the diameter change. When your speedo reads a certain speed, the actual speed is adjusted by the ratio of new to original tyre diameter.

Formula

Actual speed = Speedo reading x (New diameter / Original diameter)
Error % = ((New diameter - Original diameter) / Original diameter) x 100

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Actual figures may vary based on driving conditions, vehicle specifications, and other factors.