Tire Calculator
Decode tire sizes like 205/55 R16, understand each number, and compare two tire sets with speedometer and ground-clearance impact.
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Reading a tire size
205 / 55 R16 means: 205 mm section width, 55% aspect ratio (the sidewall is 55% of the width), R = radial construction, 16 inches rim diameter. Load and speed indexes follow the size.
- Section width : tread width in mm
- Aspect ratio : sidewall height as % of width
- Rim diameter : wheel diameter in inches
- Load index : max load the tire can carry
- Speed index : max certified speed
Changing size
When you swap to a bigger wheel you must pick a tire that keeps the total diameter within about 3% of the original, otherwise the speedometer reads wrong, ABS/ESP can misbehave and the bodywork may rub.
FAQ
Yes. A larger diameter means the car travels further per revolution, so the speedometer under-reads. The calculator shows the percentage difference.
Lower profile, wider tire on a one-inch bigger rim while keeping the same total diameter. E.g. 205/55 R16 → 225/45 R17. It improves looks and grip but firms up the ride.
Keep the new diameter within ±3% of the original, ideally under 2%, and keep the load index at or above the original requirement.
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