Tire Size Calculator
Enter a P-metric tire size to find its sidewall height, overall diameter, circumference and revolutions per mile. Add a second size to see speedometer error.
How to use the tire size calculator
- Enter the width, aspect ratio and rim diameter, e.g. 225 / 45 / 17.
- Read the sidewall height, overall diameter and revs per mile.
- Optionally enter a second size to compare diameter and speedometer error.
Examples
Read a 225/45R17
width 225, aspect 45, rim 17
24.97 in diameter, 807.6 revs/mile
Compare for speedo error
225/45R17 vs 235/55R17
about +8.8% speedometer error
Frequently asked questions
How do I read a tire size like 225/45R17?
225 is the section width in millimetres, 45 is the sidewall height as a percent of that width, R means radial, and 17 is the rim diameter in inches.
How is overall tire diameter calculated?
Sidewall height is width times aspect ratio over 100. Convert two sidewalls to inches and add the rim diameter: rim + 2 x sidewall / 25.4.
What is revs per mile?
It is how many times the tire rotates to cover one mile, found by dividing 63360 inches per mile by the tire circumference in inches. Smaller tires turn more often.
How does tire size affect my speedometer?
A taller tire travels farther per turn, so the speedometer reads low and the odometer undercounts. This tool reports the percent error between two sizes.
Can I fit a different size on the same wheel?
Often yes, if overall diameter stays within a few percent and the width suits the rim. Always check clearance and load rating before changing tire size.
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