Decode tire sizes like 225/45R17 into diameter, sidewall and circumference, and compare two sizes to see the speedometer difference.
About this tool
Decode tire sizes and compare diameter, sidewall and speedometer difference.
How to use Tire Size Calculator
- Enter the tire size printed on the sidewall.
- Add a second size to compare.
- Read the dimensions and the difference in diameter and speed.
Worked example
Going from 225/45R17 to 235/40R18 increases the diameter slightly; at an indicated 60 mph you travel a little faster.
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.
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