Tire Size Calculator
Decode tire sizes and compare diameter, sidewall and speedometer difference.
Enter your values to see the result.
Continue your work
- Speed Converter — Convert mph and km/h.
- Length Converter — Convert mm and inches.
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Clear instructions
Find steps, examples and limitations below.
Use online
Open the tool in a supported web browser.
Free to use
No sign-up required. Tool-specific limits may apply.
How to use the Tire Size Calculator
Decode tire sizes and compare diameter, sidewall and speedometer difference.
- 1 Enter the tire size printed on the sidewall.
- 2 Add a second size to compare.
- 3 Read the dimensions and the difference in diameter and speed.
Example and practical tips
Going from 225/45R17 to 235/40R18 increases the diameter slightly; at an indicated 60 mph you travel a little faster.
Frequently asked questions
What do the numbers mean?
In 225/45R17, 225 is the width in mm, 45 the sidewall as a percentage of the width, and 17 the wheel diameter in inches.
How much difference is safe?
Staying within about 3% of the original diameter is the usual guideline.
Report an issue
Something broken or not quite right? Tell us and we will look into it.