Tire Size Calculator
Enter your current and new tyre sizes to see the difference in overall diameter and what your speedometer will read. A size like 225/65R17 means 225 mm wide, a sidewall 65% of the width, and a 17-inch wheel.
Your details
Current tyre
New tyre
Result
Speedometer error
-7.7%
When your speedometer reads 60 mph, you are actually going 55.4 mph
- Old diameter
- 28.52"
- New diameter
- 26.33"
- New sidewall
- 4.16"
- Reads 30 mph
- 27.7 mph
- Reads 60 mph
- 55.4 mph
- Reads 100 mph
- 92.3 mph
- The diameter difference is over 3%: expect a noticeable speedometer/odometer error and possible ABS or gearing effects.
Good to know: When your speedometer says 60 mph, your true speed with the new tyres is shown above.
About this tool
Compare two tire sizes to see differences in diameter, sidewall height and width, and how much your speedometer and odometer readings shift.
Formula
Tire diameter = rim + 2 × sidewall, where sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4. Speedometer error % = (new ÷ old − 1) × 100.
Reference values
- 235/45R18 diameter ≈ 26.3 in
- 225/65R17 diameter ≈ 28.0 in
- Keep diameter within ±3% of stock
Questions people ask
How do I read a tyre size?
In 225/65R17, 225 is the width in mm, 65 is the sidewall height as a percentage of the width, R means radial, and 17 is the wheel diameter in inches.
How much diameter change is acceptable?
Many mechanics advise keeping it within about 3% of the original to limit speedometer, ABS and gearing effects.
Will bigger tyres make my speedometer read low?
Yes. A larger overall diameter covers more ground per turn, so the speedometer under-reads your true speed.
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How we calculate this
We use the standard published formula or definition for this calculation, and we show the working so you can check it.
Spotted a mistake? Tell us and we'll check it. How we check our calculators