Speedometer Error Calculator
See how a tire-size change can affect your actual road speed.
Use this Speedometer Error Calculator to estimate how a change in tire diameter may affect your actual road speed. Enter your original tire diameter, new tire diameter and indicated speed to compare your speedometer reading with the estimated actual speed.
Speedometer Results
Common Speed Comparison
Enter your tire diameters and speedometer reading above to estimate the error.
How to Use the Speedometer Error Calculator
This tool helps estimate how changing tire diameter can affect the difference between your speedometer reading and actual road speed.
Enter your original tire diameter, new tire diameter and indicated speed. The calculator will estimate your actual speed, speed difference and percentage of speedometer error.
Why Tire Size Affects Speedometer Accuracy
A larger tire travels farther with each revolution, while a smaller tire travels a shorter distance. Because many vehicle speedometers are calibrated for the original tire diameter, changing tire size can cause the indicated speed to differ from the vehicle’s actual road speed.
This comparison is especially useful when changing wheel and tire combinations, installing larger or smaller tires, or checking how a modified tire diameter may influence everyday driving. For the most accurate result, use the actual measured or manufacturer-listed tire diameter whenever possible.
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