Speedometer Error Calculator
Changing tire size affects your speedometer reading. Enter your original and new tire sizes — plus your indicated speed — to see the real impact.
How It Works
Speedometer Error = (newDiameter / oldDiameter − 1) × 100%
A larger tire rotates fewer times per mile, so the speedometer (which counts rotations) reads lower than actual speed. A smaller tire causes it to read higher.