Speedometer error after tire change
The speedometer is calibrated to the original tire diameter, so a new diameter changes your real speed at the same reading. This tool works out the gap.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter original tire width, original aspect ratio, original rim diameter, new tire width, new aspect ratio and new rim diameter.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
True speed = speedometer reading × new overall diameter ÷ original overall diameter, where diameter = rim × 25.4 + 2 × width × aspect ÷ 100.
Example
Inputs: Original tire width = 205 mm, Original aspect ratio = 55 %, Original rim diameter = 16 in, New tire width = 225 mm, New aspect ratio = 45 %. Result: True speed: 100.4 km/h; Difference from reading: 0.38 km/h; Difference in percent: 0.38%.
Limits and caveats
- It assumes the speedometer was exact on the original tires, while most speedometers read slightly high by design.
- Tread wear reduces the diameter by a few millimeters.
Questions about this tool
Is the odometer affected too?
Yes, by the same ratio. A tire 2% larger makes the odometer log about 2% less distance than you really drive.
Why does a new car read higher than my real speed?
UN Regulation 39 forbids speedometers from showing less than the true speed, so makers set them to read slightly high.
How can I check my real speed?
Compare the speedometer with GPS speed on your phone on a level road at a steady speed.