Calculate tire diameter, sidewall and circumference from the size (205/55 R16) and compare two sizes, with the speedometer difference. Enter your values; the result updates as you type, with the method shown below.
About this calculator
A tyre size such as 205/55 R16 gives the width in millimetres, the sidewall height as a percentage of the width and the rim diameter in inches. This calculator turns it into an overall diameter and compares two sizes.
Enter the current size and the size you are considering. The calculator shows both diameters, the sidewall height, the circumference, the difference in percent and what the speedometer really shows.
When to use it
- Choosing winter tyres or bigger rims.
- Checking that a new size fits.
- Understanding a speedometer error.
- Comparing tyre offers.
How to read the result
Diameter = rim × 25.4 + 2 × width × ratio / 100. A 205/55 R16 is about 632 mm across.
Stay within about ±3% of the original diameter, and check the sizes approved for your car.
Mistakes to avoid
- Changing the diameter too much.
- Ignoring the load and speed index.
- Fitting a size not approved for the car.
Formula
Diameter = rim × 25.4 + 2 × width × aspect ratio
Worked example
205/55 R16: 632 mm diameter; 225/45 R17: 634 mm, +0.38%.
Frequently asked questions
How far can I change the size?
Stay within about ±3% of the original diameter.
What does 205/55 R16 mean?
Width 205 mm, sidewall 55% of the width, 16-inch rim.
What does 205/55 R16 mean?
205 mm wide, sidewall 55% of the width, 16-inch rim.
Why does the speedometer change?
A different diameter changes the distance per wheel turn.
Can I fit any size?
Only sizes approved for your vehicle.
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Results are estimates for information only. Check important figures with a professional.