Tire Size Calculator
Compare Tire Diameter & Speedometer Accuracy
Ready for Calculation
Enter your original and new tire sizes to compare diameter and speedometer accuracy.
Reading Tire Size Codes
A tire size like 225/50R17 packs three measurements into one code: section width in millimeters (225), aspect ratio as a percentage of that width (50%), and rim diameter in inches (17). Changing any of these numbers changes the tire's overall diameter — and your car's effective gearing.
Section Width & Aspect Ratio
Aspect ratio is the sidewall height as a percentage of the section width — a "50" series tire has a sidewall that's 50% as tall as the tire is wide.
Speedometer Accuracy
Your speedometer is calibrated for a specific tire diameter. A larger overall diameter means your car actually travels faster than the speedometer indicates — and your odometer under-reads distance.
Why "Plus Sizing" Matters
"Plus sizing" means moving to a larger wheel diameter while lowering the aspect ratio to keep overall tire diameter close to the original. Most tire professionals recommend staying within about 3% of your original overall diameter to avoid throwing off your speedometer, ABS, and traction control calibration.
Stay Within 3%
A diameter change beyond roughly ±3% can meaningfully affect handling, ground clearance, and electronic driver-assist calibration.
Key Takeaways
- Bigger Tire, Faster Real Speed: A larger new diameter means your actual speed at a given indicated speed is higher than shown.
- Revolutions per Mile: A larger diameter tire turns fewer times per mile, which is why odometers can under-read after a tire size change.
- Check Fitment First: This calculator covers the math only — always verify a new size physically clears your fenders, suspension, and brakes.