Tyre Size and Diameter Calculator

Compare your current tyres with a planned size to see diameter changes, ride height differences, and the speedometer impact in plain language.

Check Your Tyre Swap

Use the standard format: width / aspect ratio R wheel diameter (all in mm/inches, e.g. 205/55R16).

How the Calculator Works

We break each tyre size into three parts: the tread width in millimeters, the sidewall height as a percentage of that width (aspect ratio), and the wheel diameter in inches. From there we calculate the total tyre diameter and how much it will change.

  • Overall diameter: Sidewall height on both sides plus the wheel diameter (converting inches to millimeters).
  • Ride height change: Half of the diameter change, since only the radius affects ground clearance.
  • Speedometer error: A larger tyre makes the speedometer read slightly slow; a smaller tyre makes it read slightly fast.

Tips for Picking the Right Size

  1. Stay close to stock diameter: Aim to keep total diameter within 2-3% to avoid big speedometer errors or clearance issues.
  2. Check clearance: Wider or taller tyres can rub on fenders, liners, or suspension components.
  3. Match load and speed ratings: Ensure the new tyre meets or exceeds the vehicle's requirements.
  4. Recheck pressure: Verify recommended pressures after changing sizes for the best wear and ride quality.