Tyre Size and Speedometer Error Calculator
If you fit tyres of a different size, your speedometer and odometer will no longer be accurate. Enter your original tyre size and the new size to see how the overall diameter changes, and what your speedometer will read.
How to enter a size
Type the size as it appears on the sidewall, for example 205/55R16 or LT265/65R17. For larger off-road sizes, such as 33x12.50R20, the first number is the overall height in inches.
How it works
The overall diameter is the wheel diameter plus twice the sidewall height. The sidewall height is the width multiplied by the aspect ratio. Your speedometer counts wheel turns, so a taller tyre covers more road per turn, and the speedometer reads lower than your real speed. A smaller tyre does the opposite.
What the result means
- Within 3 percent: usually acceptable.
- 3 to 5 percent: noticeable. Check clearance, and consider recalibrating the speedometer.
- Over 5 percent: large. It can affect speedometer accuracy, braking and stability systems, gearing and clearance. Ask a tyre shop for advice first.
Whatever the size, the tyre must meet your vehicle's required load index and speed rating.