Tire size comparison calculator
Before changing tire or rim size, enter the current and new sizes to see the diameter difference, which is commonly kept within about 3%.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter current tire width, current aspect ratio, current rim diameter, new tire width, new aspect ratio and new rim diameter.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Sidewall (mm) = width × aspect ratio ÷ 100. Overall diameter = rim × 25.4 + 2 × sidewall. Circumference = π × diameter; revs per km = 1,000,000 ÷ circumference in mm.
Example
Inputs: Current tire width = 205 mm, Current aspect ratio = 55 %, Current rim diameter = 16 in, New tire width = 225 mm, New aspect ratio = 45 %. Result: Overall diameter difference: 0.38%; Current overall diameter: 631.9 mm; New overall diameter: 634.3 mm.
Limits and caveats
- Figures are nominal for an unloaded tire; real diameter varies slightly by brand, pressure and wear.
- Check rim width compatibility and fender clearance before buying.
Questions about this tool
How do I read 205/55R16?
205 is the width in mm, 55 is the sidewall height as a percent of width, R means radial construction, and 16 is the rim diameter in inches.
How much diameter difference is acceptable?
A common industry rule is to stay within about 3% up or down so the speedometer, ABS and stability control are not thrown off much.
What happens with a larger diameter?
The speedometer reads lower than your real speed, engine rpm drops at the same speed, and acceleration can soften slightly.