Tire Size Calculator
Calculate tire diameter, sidewall height, circumference, revolutions per mile, and speedometer error when changing tire sizes.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to Use the Tire Size Calculator
This tool calculates key tire dimensions and speedometer accuracy. Enter values in the specified units.
- Tire Width (mm): Enter the tire width in millimeters (e.g., 225).
- Aspect Ratio (%): Enter the sidewall height as a percentage of width (e.g., 45).
- Wheel Diameter (inches): Enter the rim diameter in inches (e.g., 17).
- Speedometer Reading (mph) — optional: Enter your indicated speed to check accuracy.
- Click Calculate to see results.
Outputs explained:
- Sidewall Height: Tire sidewall in inches.
- Overall Diameter: Total tire height in inches.
- Circumference: Distance per revolution in inches.
- Revolutions per Mile: How many times the tire rotates per mile.
- Actual Speed & Error: If you entered a speedometer reading, shows true speed and percentage error (positive = speedometer reads low).
All inputs must be positive numbers. Errors appear in red if missing.
Previous Results
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Frequently Asked Questions
Tire size is typically formatted as P225/45R17, where 225 is the width in mm, 45 is the aspect ratio (sidewall height as a percentage of width), and 17 is the wheel diameter in inches.
Tire diameter = wheel diameter + 2 * (tire width * aspect ratio / 100) / 25.4, where the sidewall is converted from mm to inches.
A larger tire diameter reduces speedometer reading (actual speed is higher), while a smaller diameter increases speedometer reading (actual speed is lower). The error can be calculated as a percentage.