Tire Size Calculator
Calculate tire diameter, width, sidewall height and circumference from a tire size like 225/45R17 for comparison and fitment.
Last reviewed by the Radiatus Cloud team
Calculate tyre dimensions from a tire size like 225/45R17.
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Calculate tyre dimensions
A tyre size such as 225/45R17 encodes three numbers: the section width in millimetres, the aspect ratio as a percentage, and the rim diameter in inches. This calculator turns those into the physical dimensions you actually need, including the sidewall height, the overall tyre diameter, the circumference and the number of revolutions per mile. The sidewall height is the width multiplied by the aspect ratio, and the overall diameter adds two sidewalls to the rim diameter.
For a 225/45R17 tyre, the overall diameter works out to about 634 millimetres, or just under twenty-five inches.
Comparing and changing tyre sizes
Understanding tyre dimensions matters when comparing sizes or fitting different tyres, because a change in overall diameter affects speedometer accuracy, gearing and clearance. A larger diameter makes the speedometer read low and alters the effective gear ratio, while a smaller one does the opposite. Keeping the overall diameter close to the original, within a small percentage, keeps these effects negligible, which this calculator helps you check.
The revolutions-per-mile figure is useful for comparing how two tyres affect speedometer and odometer readings. All calculation happens locally in your browser.
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Frequently Asked Questions
What do the numbers in a tyre size mean?
The first is the section width in millimetres, the second is the aspect ratio as a percentage of the width, and the last is the rim diameter in inches.
How is the overall diameter calculated?
Add two sidewall heights to the rim diameter. The sidewall is the width times the aspect ratio, converted to the same units as the rim.
Why does tyre diameter matter when changing sizes?
A different overall diameter changes speedometer accuracy, effective gearing and clearance, so keep it close to the original size.
What is revolutions per mile?
It is how many times the tyre rotates per mile, which depends on circumference and affects speedometer and odometer readings.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter the tire width, aspect ratio and rim diameter.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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