Understanding the Speedometer Correction Calculator
Changing tire or wheel size alters how far your car travels per wheel revolution, which makes your speedometer read incorrectly. This calculator shows the true speed for a given indicated speed after a tire change, so you know how far off your gauge is.
Why the error happens
Your speedometer is calibrated for the original tire diameter. Fit larger tires and the car actually travels farther per revolution than the gauge assumes, so your true speed is higher than indicated. Smaller tires create the opposite error. The size of the error grows with how much the diameter differs from stock.
Correcting it
For small tire changes the error may be minor, but larger swaps can be significant enough to matter for speeding tickets and safety. Many vehicles can be recalibrated electronically to match the new tire size; otherwise, knowing the percentage error lets you mentally adjust your gauge reading until it is fixed.
Tips & things to know
- •Larger tires make actual speed higher than indicated.
- •Bigger size changes cause bigger errors.
- •Many cars can be recalibrated to the new tire size.