Speedometer Error Calculator
Calculate speedometer error from tire size changes
About this calculator
The Speedometer Error Calculator determines how changing your tire size affects your speedometer accuracy. When you install larger or smaller tires than factory specifications, your speedometer will display incorrect speeds because it's calibrated for the original tire circumference. This calculator helps you understand the percentage error and actual speed differences, which is crucial for legal compliance, fuel economy tracking, and maintaining accurate odometer readings.
How to use
Enter your original tire size in the standard format (width/aspect ratio/wheel diameter). Then input your new tire size in the same format. The calculator will show the percentage error and how your actual speed differs from what your speedometer displays, helping you adjust your driving accordingly.
Frequently asked questions
Why does tire size affect speedometer accuracy?
Speedometers calculate speed based on wheel rotations. Larger tires cover more distance per rotation, making the speedometer read slower than actual speed.
How much speedometer error is acceptable?
Most experts recommend keeping speedometer error within 3-5%. Larger errors can affect legal compliance and vehicle systems like ABS and traction control.
Do larger tires make my speedometer read high or low?
Larger tires make your speedometer read lower than your actual speed, while smaller tires make it read higher than reality.