Speedometer Error Calculator
Enter your indicated speed and old & new tire diameters.
Result
How to use
- Enter your values in the fields above.
- Press Calculate to see your result instantly.
- Use the Share button to copy a link to your result.
About this calculator
A car's speedometer is calibrated to the original tire diameter specified by the manufacturer, since the gauge actually measures wheel rotation speed and converts it to road speed using that assumed diameter. Fitting a different-sized tire or wheel changes how far the car actually travels per rotation, so the speedometer keeps displaying speed based on the old diameter — this calculator applies the ratio of new to old tire diameter to your indicated speed to find your true road speed and the resulting error.
This calculation matters to car enthusiasts and off-road drivers who upsize or downsize wheels and tires for looks, off-road clearance, or handling changes, since driving faster than the speedometer shows can mean unknowingly exceeding a speed limit, and the same diameter shift also throws off the odometer and any speed-dependent electronics tied to wheel-speed sensors.
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