Find tire dimensions and compare two sizes for speedometer error.
Calculated instantly in your browser.
How do you calculate tire size dimensions?
Sidewall = width × aspect ratio ⁄ 100, and overall diameter = rim diameter (mm) + 2 × sidewall. A tire size like 225/45R17 reads as width (mm) / aspect ratio (%) R rim (inches). For example, 225/45R17 has a 101 mm sidewall and a 634 mm (24.97 in) overall diameter. A diameter change of x% throws the speedometer off by x%.
Understanding your result
A tire size is written as width (mm) / aspect ratio (%) R rim diameter (inches). Changing the size alters the rolling diameter, which throws the speedometer off and can affect clearance and gearing. Keep the new diameter within about 3% of the original.
Formula and method
Sidewall = width × aspect ratio ⁄ 100. Overall diameter = rim diameter (mm) + 2 × sidewall. A diameter change of x% makes the speedometer read off by x%.
Worked example
225/45R17 has a 101 mm sidewall and a 634 mm (24.97 in) overall diameter.
How to use this tool
- Enter your tire size (e.g. 225/45R17).
- Optionally enter a size to compare.
- Read the dimensions and any speedometer error.
Common mistakes to avoid
- Changing diameter by more than ~3%, which affects the speedometer and clearance.
- Confusing the aspect ratio (a percentage) with a millimetre value.
About the Tire Size Calculator
The Tire Size Calculator reads a tire size like 225/45R17 and works out its sidewall height, overall diameter, circumference and revolutions per mile. Add a second size to see the speedometer error from a change.
Who should use this tool
Drivers, car enthusiasts and anyone changing wheel or tire sizes.
Benefits
- Overall diameter in mm and inches.
- Sidewall, circumference and revs per mile.
- Compare two sizes for speedometer error.
- Reads the standard size format.
Practical use cases
- Checking if a new tire size fits.
- Estimating speedometer error after a change.
- Comparing plus-sizing options.
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Frequently asked questions
What does 225/45R17 mean?
A 225 mm wide tire with a sidewall 45% of that width, on a 17-inch rim.
How much can I change tire size?
Keep the overall diameter within about 3% of the original to avoid speedometer error and clearance problems.