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Tire size calculator

Decode a tyre size like 205/55 R16 — width in mm, sidewall height as a percentage, rim in inches — and compare two sizes to see the real diameter difference and what it does to your speedometer.

How to read it
205Width

Tread width in millimetres, measured sidewall to sidewall.

55Aspect ratio

Sidewall height as a percentage of the width — 55 means the sidewall is 55% of 205 mm.

R16Rim

R for radial construction, 16 for the rim diameter in inches.

This calculator is pure geometry. Before actually fitting a different size, also check load index, speed rating, wheel-arch clearance and what your vehicle papers allow — a tyre can fit the math and still not fit the car. The rule of thumb: stay within ±3% overall diameter.
Frequently asked

What does 205/55 R16 mean?

205 is the tread width in millimetres, 55 means the sidewall is 55% of that width, R stands for radial construction and 16 is the rim diameter in inches.

Can I fit a different tire size on my car?

Geometrically, stay within about ±3% of the original overall diameter. But also check the load index, speed rating, wheel-arch clearance and what your vehicle documents allow.

How does tire size affect my speedometer?

The speedometer counts wheel revolutions. A larger-diameter tire covers more ground per turn, so the speedo under-reads — this calculator shows exactly by how much.