Speedometer error calculator
A speedometer counts wheel turns, so a different tyre or a drifting calibration changes what it shows. Compare the old and new tyre size, or enter the dash reading next to a GPS reading, to get the error and the true speed.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose tyre sizes or GPS.
- Enter the speed your speedometer shows.
- For tyres, enter the original and the new size; for GPS, enter the GPS speed at the same moment.
- Read the true speed and the error in percent.
How it works
- Tyres: true speed = reading × new diameter ÷ original diameter, with each diameter = rim × 25.4 + 2 × width × aspect ratio ÷ 100 (mm).
- GPS: the true speed is the GPS speed.
- Error = (reading − true speed) ÷ true speed × 100.
- True speed at 100 = 100 × true speed ÷ reading, a handy way to scale any reading.
Worked example
Changing from 225/45R17 (634.3 mm) to 225/40R18 (637.2 mm): at a reading of 100 the true speed is 100 × 637.2 ÷ 634.3 = 100.46, so the speedometer is off by −0.46% – it reads slightly low. With GPS: the dash shows 100 while GPS shows 95, so the error is (100 − 95) ÷ 95 = +5.26% and a dash 100 means a true 95.
Tips
- Compare at steady speed. Accelerating makes the dash and GPS lag differently.
- Odometer follows the same calibration. A larger tyre means more real distance than the odometer shows.
- Check the local rules. Some countries limit how far a speedometer may differ from true speed; look up yours.
Frequently asked questions
How does a different tyre size change the speedometer?
The speedometer is calibrated for the original rolling circumference. A bigger tyre travels further per turn, so you go faster than shown; a smaller one makes the dash read high. The true speed is the reading × new diameter ÷ original diameter.
What does a negative error mean?
Error = (shown − true) ÷ true. A negative value means the speedometer shows less than your true speed, which is what a larger tyre causes. A positive value means it reads higher than the truth.
Is GPS speed a reliable reference?
Phone and satellite navigation speed is usually close to true speed on open roads, since it comes from position changes rather than wheel turns. Use a steady speed on a straight road, and avoid tunnels and tight city streets.
Can I enter mph or km/h?
Either, as long as both values use the same unit. The error and the percentage do not depend on the unit.