Tire Size Conversion Tool
Convert metric tire sizes to inches, calculate diameter and circumference, estimate revs per mile, and compare two sizes for speedometer and clearance changes.
Current Tire (A)
New Tire (B) - Optional Comparison
How this tire size conversion calculator helps
Changing tire size affects more than appearance. It changes your effective gearing, speedometer accuracy, fuel economy, and even available fender clearance. This calculator gives you a practical tire size comparison in one place so you can make a smarter decision before buying wheels and tires.
Use it as a metric tire size calculator, a tire diameter calculator, or a tire circumference calculator. If you enter two sizes, it also acts as a plus sizing checker to estimate how close your new setup is to stock.
How to read a tire size
Example: 225/45R17
- 225 = section width in millimeters.
- 45 = aspect ratio (sidewall height is 45% of section width).
- R = radial construction.
- 17 = wheel diameter in inches.
From this format, we can derive sidewall height, overall tire diameter, circumference, and revolutions per mile. Those values make it much easier to compare tire options objectively.
What the calculator converts
- Section width (inches): quick metric-to-inches conversion.
- Sidewall height: shown in both mm and inches.
- Overall tire diameter: key value for fitment and speedometer behavior.
- Circumference: useful for gearing and distance calculations.
- Revolutions per mile: helps estimate drivetrain and speedometer impact.
- Approximate flotation format: simplified format like 25.0x8.9R17.
Why tire diameter matters
Diameter is the number that usually matters most when changing size. A larger tire travels farther per revolution, so your vehicle moves faster than the speedometer indicates. A smaller tire does the opposite.
Practical effects of diameter changes
- Speedometer and odometer error
- Acceleration feel and effective gearing
- Cruising RPM at highway speed
- ABS and traction-control behavior (if difference is large)
- Ground clearance and potential rubbing
A common guideline: stay within ±3%
Many enthusiasts and tire shops treat a diameter change within about 3% as a reasonable target for daily driving compatibility. Staying closer to stock generally helps preserve OEM speedometer behavior and drivetrain calibration.
That said, fitment still depends on wheel width, offset, suspension height, tire model, and alignment. Always physically verify clearances when you are near the limit.
Formula reference
Core equations used in this page
- Sidewall height (mm) = width × (aspect ratio ÷ 100)
- Overall diameter (mm) = (2 × sidewall height) + (wheel diameter in × 25.4)
- Overall diameter (in) = diameter (mm) ÷ 25.4
- Circumference (in) = π × diameter (in)
- Revs per mile = 63,360 ÷ circumference (in)
These equations are standard and widely used across tire size charts, fitment tools, and wheel/tire calculators.
Tips before you buy
- Compare your planned size with your current or factory size.
- Check wheel width recommendations for your selected tire model.
- Confirm offset and suspension/fender clearance at full steering lock.
- Consider winter vs. summer tire behavior (compound and tread matter too).
- Recalibrate speedometer if diameter change is significant.
Final thought
A tire size conversion calculator gives you a strong starting point, not a final fitment guarantee. Use the numbers here to narrow options quickly, then confirm with your vehicle specs and a trusted tire installer. A few minutes of math can save you a costly setup mistake.