Compare two different tire sizes to instantly calculate diameter, width, sidewall height, and speedometer error percentage before buying new wheels.
Tire Size Calculator
Compare two tire sizes to see exactly how the new setup will affect your speedometer reading, ride height, and clearance.
Input Specs
Comparison Result
How to Read a Tire Size
Upgrading wheels or buying new tires can be complicated if you don't know how to decode the metric tire sizing system printed on the side of your wheel. To use our calculator, you need the three core numbers printed on your tire wall, which typically look like 225/45R17.
Width
The width of the tire measured in millimeters from sidewall to sidewall.
Aspect Ratio
The height of the sidewall as a percentage of the width. Here, the height is 45% of 225mm.
Wheel Size
The diameter of the wheel (the metal rim) that the tire is designed to fit onto, measured in inches.
Speedometer Error
Your car's internal computer calculates your vehicle's speed by counting how many times your wheels rotate. It assumes you are running the Original Equipment Manufacturer (OEM) factory tire size.
If you install tires with a larger overall diameter, a single rotation travels further on the road. Consequently, your speedometer will read slower than your actual physical speed, potentially resulting in speeding tickets. Our tool calculates this exact discrepancy.
The 3% Safety Rule
When modifying your vehicle's stance or wheel setup, experts universally recommend keeping the overall diameter difference within ±3% of the factory original size.
Exceeding this 3% threshold can interfere with vital safety systems that rely on precise wheel speed sensors, primarily your Anti-Lock Braking System (ABS) and Electronic Stability Control (ESC), while also risking mechanical rubbing against the wheel well when turning.
Frequently Asked Questions
Can I put wider tires on the same rims?
Yes, within reason. Rims typically have an acceptable width range. Going 10mm or 20mm wider usually fits securely, but if you increase the width, you often need to decrease the aspect ratio to maintain the original overall diameter. If you go too wide, the tire will "bulge" or pop off the bead.
How is tire circumference calculated?
First, we convert the width to inches and multiply it by the aspect ratio to find the sidewall height. We multiply the sidewall by two and add it to the wheel diameter to find the total height. Finally, we multiply that total height by Pi (3.14159) to find the circumference.
The tools and calculators provided on The Simple Toolbox are intended for educational and informational purposes only. They do not constitute financial, legal, tax, or professional advice. While we strive to keep calculations accurate, numbers are based on user inputs and standard assumptions that may not apply to your specific situation. Always consult with a certified professional (such as a CPA, financial advisor, or attorney) before making significant financial or business decisions.
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