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