Tire Size Calculator
Compare tyre sizes and see the effect on diameter and speedometer reading.
About the Tire Size Calculator
In a size like 225/45R17, 225 is the width in mm, 45 the sidewall height as a percentage of width, R means radial and 17 the rim diameter in inches. Overall diameter = rim × 25.4 + 2 × width × aspect ÷ 100. A larger diameter makes the speedometer read low. Most guidance keeps replacements within about 3% of the original diameter.
The formula
Diameter (mm) = rim × 25.4 + 2 × width × aspect%
Worked example
Example: Diameter difference = +2.36%
- Inputs: Current tyre size: 205/55R16; New tyre size: 215/50R17; Speedometer reading: 100.
- Formula: Diameter (mm) = rim × 25.4 + 2 × width × aspect%
- Result: Diameter difference = +2.36%
Questions people ask
Is it legal to change tyre size?
Usually, if load and speed ratings and clearance are met; check local rules and insurance.
Does it affect ABS and stability control?
Large changes can; stay close to the original diameter.