Tire Size Calculator
Compare two tire sizes to see the change in diameter, circumference and speedometer reading before you buy.
Diameter difference
+2.1%
- Current diameter
- 724 mm
- New diameter
- 739 mm
- Difference
- +15 mm
With the new size, when your speedometer reads 100 mph you'd actually be going 102.1 mph.
Keep diameter within about ±3% of stock to avoid speedometer error, rubbing and ABS/traction issues. 225/65R17 → 235/60R18.
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How to use this calculator
Enter your current tire size and the new size you're considering, each in the standard format like 225/65R17. The calculator shows the overall diameter of each, the difference in millimetres and percent, and how much your speedometer would be off with the new size.
Aim to keep the diameter within ±3% of stock. Bigger differences can cause rubbing and throw off your speedometer, odometer and safety systems.
Frequently asked questions
- How do I read a tire size like 225/65R17?
- 225 is the tread width in millimetres, 65 is the aspect ratio (sidewall height as a percent of width), R means radial, and 17 is the wheel diameter in inches. Overall tire diameter ≈ wheel size plus twice the sidewall height.
- How much bigger or smaller can I go on tire size?
- Stay within about ±3% of the original overall diameter. Beyond that you risk speedometer error, rubbing on the fender or suspension, and incorrect ABS, traction-control and odometer readings.
- How do bigger tires affect my speedometer?
- A larger-diameter tire travels farther per revolution, so your speedometer reads slower than your real speed (and your odometer under-counts). A smaller tire does the opposite. This calculator shows your true speed at an indicated 100.
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