Tire Size Calculator

A tire marked 205/55 R16 encodes three numbers: 205 mm wide, sidewall height of 55% of the width, for a 16-inch rim. This tire size calculator decodes any marking into the dimensions that actually matter — overall diameter, sidewall height and circumference.

Use it before changing wheel or tire sizes: keeping the overall diameter within ±3% of stock preserves your speedometer accuracy, gearing and clearance.

Enter valid values to see the results.

Formula

Diameter = Rim × 25.4 + 2 × (Width × Aspect ÷ 100), all in mm

The aspect ratio is a percentage of the width, not a millimetre figure — a 205/55 and a 225/55 have different sidewall heights. Circumference = π × diameter; revolutions per km = 1,000,000 ÷ circumference in mm.

What you need

Read the marking on the tire sidewall: e.g. 205/55 R16 means width 205, aspect 55, rim 16.
Width is in millimetres, rim diameter in inches — the calculator converts automatically.
When comparing two sizes, run both and compare overall diameters: stay within ±3% of the original.

Worked example

A 205/55 R16: sidewall = 205 × 0.55 = 112.75 mm; diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm (24.9 in); circumference ≈ 1.985 m; about 504 revolutions per kilometre. Swapping to 225/45 R17 gives 634.3 mm — only +0.4%, a safe swap.

Practical tips

Plus-sizing (bigger rim, lower profile) sharpens handling but harshens the ride and raises wheel damage risk on potholes.
A larger overall diameter makes the speedometer read slower than your real speed — and vice versa.
Check clearance at full steering lock and full suspension compression before going wider.
Keep all four tires the same size on AWD vehicles — mismatched diameters strain the drivetrain.

Common tire sizes and overall diameter

Size	Diameter	Sidewall
195/65 R15	634.5 mm	126.8 mm
205/55 R16	631.9 mm	112.8 mm
225/45 R17	634.3 mm	101.3 mm
225/40 R18	637.2 mm	90.0 mm

Common mistakes

1 Reading the aspect ratio as millimetres — it is a percentage of width.
2 Comparing rim diameters instead of overall diameters when judging fit.
3 Ignoring load and speed ratings printed after the size (e.g. 91V) when choosing replacements.

Frequently asked questions

What does 205/55 R16 mean?

205 mm tread width, sidewall height equal to 55% of the width (112.75 mm), radial construction, for a 16-inch rim. Overall diameter is about 632 mm.

How much can I change tire size safely?

Keep the overall diameter within ±3% of the factory size. Beyond that, speedometer error, gearing changes and clearance issues appear.

Will bigger wheels hurt fuel economy?

Usually slightly: larger, wider wheels are heavier and increase rolling resistance and aerodynamic drag. Expect a few percent higher consumption.

How does tire size affect the speedometer?

Speedometers count wheel revolutions. A tire with a 3% larger diameter travels 3% further per revolution, so at a indicated 100 km/h you actually drive 103 km/h.