Car & Vehicle

Tyre Speedometer Error Calculator

What a tyre size change does to the speedometer and the odometer, in km/h and kilometres.

4 inputs Free, no sign-up

Your figures

Usually the size on the door placard - what the vehicle left the factory with.

As printed on the sidewall - 205/55 R16, for example. Width in millimetres, sidewall as a percentage of the width, rim in inches.

The result shows what the odometer records while the vehicle actually covers this.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A speedometer counts wheel rotations and multiplies by a circumference it was told to assume. Change the tyre and the assumption is wrong by exactly the percentage the diameter changed.

A taller tyre turns more slowly for the same road speed, so the needle reads low — you are going faster than it says. A shorter tyre reads high. This page gives the size of the error, and what it does to the odometer over a distance.

How to use this calculator

  1. Enter the size the vehicle was calibrated for — the one on the door placard.
  2. Enter the size actually fitted.
  3. Set a distance to check the odometer over. A thousand kilometres makes the error easy to read.

The formula

Error %            = (New diameter - Old diameter) / Old diameter x 100
Indicated at 60    = 60 x Old diameter / New diameter
Odometer over D    = D x Old diameter / New diameter

Both the speedometer and the odometer are wrong by the same proportion, because both count the same rotations.

SymbolMeaningUnit
Calibrated size What the instrument assumes
Fitted size What is actually on the car

Worked example

Calibrated for 205/55 R16, fitted with 215/60 R16 — 5.14% taller:

At a true 60 km/h the speedometer reads 57.1
Over a true 1,000 km the odometer records 951

Which means at an indicated 100 you are actually doing about 105, and the service interval arrives about 5% later than it should.

Notes and limits

  • Most speedometers are deliberately calibrated to read slightly high from the factory, because regulations in many countries forbid them reading low. A taller tyre eats into that margin before it starts under-reading.
  • Reading low is the direction that matters. It means you are travelling faster than indicated, with the speed camera keeping its own count.
  • The odometer error compounds over the life of the vehicle and affects service intervals, warranty distance and resale mileage.
  • GPS speed from a phone is a reasonable independent check, though it lags and struggles under cover.
  • Whether a given error is acceptable on your vehicle is a question for the handbook, not for this page.

What this calculation assumes

  • The speedometer is calibrated exactly for the original size, which is rarely exactly true - most read slightly high from the factory.
  • Dimensions are nominal, calculated from the sizes rather than measured.

Frequently asked questions

How much does a bigger tyre affect the speedometer?

By the same percentage as the change in overall diameter. A tyre 5% taller makes the speedometer read 5% low - at a true 100 km/h it shows about 95.

Which way does the error go?

A taller tyre makes the speedometer read low, so you are going faster than shown. A shorter tyre makes it read high. The odometer moves the same way.

Can the speedometer be recalibrated?

On many modern vehicles the wheel size is a stored parameter that a dealer or a specialist can adjust. Whether it is available on yours is a question for the manufacturer.

Does this affect my fuel economy readings?

Yes - the trip computer divides by the same wrong distance, so its km/L figure is off by the same percentage. A tank measurement with a corrected distance is the reliable check.

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