Tire Size Comparator
Parse and compare tire size codes like 225/45R17 by diameter and revs
About this calculator
The Tire Size Comparator parses standard tire codes like 225/45R17 and computes the sidewall height, overall diameter in millimetres and inches, circumference, and revolutions per mile. Compare two sizes to see the diameter difference and the resulting speedometer error — a positive error means the dash reads low and you are actually travelling faster. Codes with P prefixes or dash separators are accepted; unparseable codes return an error.
How to use the Tire Size Comparator calculator
- Enter your values into the input fields.
- Read the result — it updates instantly as you type.
- Adjust the inputs to compare scenarios.
- Copy the page URL to share the exact calculation.
Common examples
- 225/45R17 → diameter 634.3 mm (24.97 in), sidewall 101.25 mm, 807.6 revs/mile
- 225/45R17 → 235/40R18: diameter +1.72%, speedometer reads 1.72% low
- 265/70R17 → 285/75R17: +7.04% diameter — the speedometer under-reads by 7.04%
- 205/55R16 alone → 631.9 mm diameter, 810.7 revs/mile
Frequently asked questions
How does the Tire Size Comparator calculator work?
Enter your numbers and the calculator applies the standard formula for this everyday task, showing the result and how it was reached. It handles the unit and rounding details so you do not have to.
When would I use the Tire Size Comparator calculator?
Whenever this comes up in daily life — planning, shopping, home projects, travel or work — and you want a quick, dependable answer instead of estimating or working it out by hand.
How accurate are the results?
The math runs at full precision internally and only the displayed value is rounded for readability. For real-world use the practical limit is the accuracy of the numbers you enter, not the calculation.
Can I share a specific calculation with someone else?
Yes. Every value you enter updates the URL, so copying the address bar and sharing the link reopens the exact same calculation for whoever clicks it.