Tire Size Calculator
Compare two tire sizes and calculate diameter difference, sidewall height, and speedometer error percentage. Free, no account needed — results in seconds.
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
What Is a Tire Size Calculator?
A tire size calculator compares two tire sizes and computes the difference in overall diameter, sidewall height, circumference, and speedometer error. You enter your current tire's size code and the replacement or upgrade size, and the tool tells you exactly how much they differ — and whether the new size is within the safe fitment range for your vehicle.
How to Use the Tire Size Calculator
- Find your current tire size. Look at the sidewall of your existing tire for a code like 225/65R17. Enter the three numbers: section width, aspect ratio, and rim diameter.
- Enter the new tire size. Input the same three numbers for the replacement or upgrade tire you want to compare.
- Review the comparison. The calculator shows the difference in overall diameter, sidewall height, circumference, and speedometer error percentage.
- Check the 3% rule. If the overall diameter difference is within ±3%, the new tire is generally safe to fit. Beyond 3%, consult a shop about wheel well clearance.
Who Is This For?
- Car enthusiasts upgrading wheels — verifying that a larger rim with a lower-profile tire maintains the same overall diameter as the factory setup, keeping the speedometer accurate.
- People comparing tire options at a shop — quickly checking whether the shop's suggested substitute tire is actually equivalent to the original before agreeing to the swap.
- Mechanics and DIYers verifying fitment — confirming that a tire size change stays within the ±3% diameter threshold before installation.
Key Benefits
- 100% private — runs entirely in your browser; no data is sent to or stored on any server.
- Completely free — no subscription, no sign-up, no paywall.
- No account required — enter two tire sizes and get results immediately.
- Speedometer error calculation — shows exactly how much your speedo will read off with the new size, which is the most important practical output for anyone fitting non-OEM tires.
Common Use Cases
An enthusiast swapping from 225/45R17 to 235/40R18 enters both sizes and sees the overall diameter difference is 0.8% — well within the safe range, and the speedometer error is less than 1 mph at highway speeds. A driver whose tire shop suggests a 215/65R16 as a substitute for their stock 205/65R16 checks the calculator and finds the overall diameter is 0.5% larger — acceptable. A truck owner considering a lift kit and larger tires calculates a 6% diameter increase and sees the speedometer error would be significant enough to require recalibration.
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 this 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 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. This calculator shows you the 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
What is a tire size calculator? ▼
Is this tire size calculator free? ▼
How do I read a tire size? ▼
Does changing tire size affect the speedometer? ▼
What is the maximum tire size I can fit? ▼
Can I put wider tires on the same rims? ▼
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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