Decode what a tire size like 205/55R16 actually means
The string on the sidewall encodes width, profile, and rim. Here is how to read it — and how much a different size would change your speedometer.
Tools used in this guide
The job
Every metric tire reads width / aspect-ratio R rim-diameter: 205/55R16 is 205 mm wide, sidewall 55 % of the width, on a 16-inch rim. Flotation sizes like 31x10.5R15 are overall diameter × width in inches, R rim.
Read or compare
Enter the size into the calculator to get overall diameter, circumference, and sidewall height. Add a second size to compare — the revolutions-per-distance difference is what bends the speedometer.
- Diameter = 2 × width × aspect% + rim × 25.4 mm.
- A taller tire makes the speedometer read low — you are going faster than it says.
Worked example
205/55R16 works out to a ~632 mm diameter and ~1,985 mm circumference; going to 225/45R17 changes the diameter by ~2.4 mm — a clean plus-size. A jump to 31x10.5R15 (~787 mm) reads roughly 20 % slow on the speedometer.
Where it stops
These are sidewall-stamp numbers; real mounted diameter shifts with brand, load, and pressure. Plus-sizing legality, load index, and clearance are separate checks this math cannot make.