Tire Size Comparison Calculator
Enter the width, aspect ratio and rim diameter for two tire sizes to compare their overall diameter and speedometer effect.
Tire Size Comparison Calculator
Compare two tire sizes to see the difference in overall diameter and the resulting effect on speedometer accuracy.
Changing tire size (e.g. going from 225/45R17 to 235/40R18) changes the overall rolling diameter, which affects speedometer and odometer accuracy. This calculator computes both diameters and the percentage difference between them.
Example
- With the default values shown above, this calculator returns: Tire 1 Overall Diameter (in) ≈ 24.97; Tire 2 Overall Diameter (in) ≈ 25.40; Diameter Difference ≈ 1.72; Speedometer Reading Difference ≈ 1.72.
Frequently Asked Questions
How is overall tire diameter calculated?
Sidewall height = width × (aspect ratio ÷ 100). Overall diameter = (2 × sidewall height converted to inches) + rim diameter.
Why does changing tire size affect my speedometer?
Your speedometer is calibrated for a specific tire diameter (via wheel rotation count). A larger diameter means each wheel rotation covers more distance than the speedometer assumes, so your actual speed will be higher than displayed, and vice versa for smaller tires.