Tire Size Calculator
Compare two tire sizes side-by-side — compatibility, diameter, speedometer error
Switching tire sizes affects more than just looks — it changes your speedometer reading, ground clearance, and fuel efficiency. Our free tire size calculator lets you compare two tire sizes side by side, showing the exact diameter difference, sidewall height, and revolutions per mile for each.
Whether you're upgrading to larger rims, running a plus-size setup, or just need a compatible replacement, this tool tells you whether the swap is safe before you spend a dollar. A difference of more than 3% in overall diameter can trigger ABS faults, affect transmission shift points, and throw off your odometer.
How to Use the Tire Size Calculator
- 1 Find your current tire size on the tire sidewall or door jamb sticker (e.g. 225/55R17).
- 2 Enter both tire sizes — current in column one, new or comparison size in column two.
- 3 Click Compare Tires — diameter, sidewall, circumference, and speedometer error calculate instantly.
- 4 Check the compatibility rating: green is safe, amber is caution, red means the size difference may cause vehicle issues.
A diameter difference under 1% is ideal. Between 1–3% is generally acceptable. Above 3% risks ABS false triggers, inaccurate speedometer readings, and potential fender rubbing.
Frequently Asked Questions
What do the three numbers in a tire size mean?
The format is Width/Aspect Ratio R Rim Diameter. For 225/55R17: 225mm wide, sidewall height is 55% of width, fits a 17-inch rim. Understanding these numbers is the first step to any safe tire swap.
How much tire size difference is safe?
Keeping overall diameter within 1–2% of the original is safest. Beyond 3% you may experience speedometer inaccuracies, interference with ABS and traction control sensors, and possible fender rubbing. Always verify with your vehicle manufacturer's specifications.
Will a different tire size affect my speedometer?
Yes. A taller tire has a larger circumference and completes fewer revolutions per mile — your speedometer will read lower than your actual speed. Our calculator shows the exact speedometer error percentage so you can account for it.
Can I use this for plus-size wheel upgrades?
Absolutely. Plus-sizing (e.g. going from 17" to 18" rims) means reducing the tire aspect ratio to keep overall diameter the same. Enter both sizes here to confirm the diameter stays within an acceptable range before purchasing.