Part of the Automotive collection — 4 tools available
Tire Size Calculator
Compare tire sizes and see the difference in diameter, width, sidewall height, and revolutions per mile. Essential before switching tire sizes on any vehicle.
About This Calculator
Upgrading or changing tire sizes affects your speedometer accuracy, effective gear ratio, fuel economy, and wheel well clearance — sometimes with expensive consequences. Even a seemingly small difference in overall diameter — say 0.7 inches — can throw off your speedometer by 2-3 mph at highway speed and change your effective gearing enough to impact acceleration and braking. The industry standard is to stay within 3% of the original overall diameter. Our tire size calculator compares your current and new tire specs side by side, showing exact differences in diameter, circumference, and revolutions per mile.
The Formula Behind This Calculator
Diameter = (Width * Aspect / 100 * 2) + Rim Sidewall = Width * Aspect / 100 Speedo error = (New / Old - 1) * 100%.
Understanding the math helps you verify results and make better decisions for your project.
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How to Use
- 1Enter your current tire width (mm), aspect ratio (%), and rim diameter (in).
- 2Enter the new tire size you're considering.
- 3Click Calculate to see the exact size comparison.
- 4A difference over 3% from stock is generally not recommended.
When to Use
- →Comparing a tire upgrade size to make sure it fits within the safe 3% diameter tolerance
- →Checking how a plus-size wheel and tire package will affect your speedometer reading
- →Evaluating tire options before buying to see the real-world impact on diameter and revolutions per mile
Tips
- ✓Keep overall diameter within ±3% of stock — larger deviations affect speedometer, ABS, and traction control
- ✓Wider tires don't always mean better grip — on wet roads, a narrower tire with proper pressure often performs better
- ✓Check clearance with the wheel turned lock-to-lock before committing to a wider tire and rim combo
FAQ
How much tire size difference is acceptable?
Stay within ±3% of the original overall diameter. Larger differences affect speedometer accuracy, ABS, and traction control systems.
Will bigger tires affect my speedometer?
Yes. Larger tires cover more ground per revolution, so your speedometer will read slower than you're actually going.
What does the aspect ratio number mean?
It's the sidewall height as a percentage of tire width. A 225/55 has a sidewall that's 55% of 225mm = 123.75mm tall.
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