Tire size calculator
On a tyre marked 205/55 R16, the 205 is the section width in millimetres, the 55 is the aspect ratio — sidewall height as a percentage of that width, so 112.75 mm — and the 16 is the wheel diameter in inches. The mixed units are a historical accident nobody has ever tidied up, and they are why the overall diameter is not obvious from the code.
On a 205/55 R16 tyre the sidewall is 55% of 205 mm, or 112.75 mm. Overall diameter is the 16 inch rim (406.4 mm) plus two sidewalls, giving 631.9 mm. A replacement size should stay within about 3% of that or the speedometer reads wrong.
How to read and compare tyre sizes
Plus-sizing — a bigger rim with a lower profile — works because the two changes cancel. Going from 205/55 R16 to 225/45 R17 keeps the overall diameter within a millimetre or two, so the speedometer, ABS and stability systems all still see what they expect. Get it wrong and everything downstream of wheel speed drifts: the speedometer, the odometer, the cruise control and, in cars that use wheel speed for it, the traction and stability thresholds. A larger diameter also makes the odometer under-read, which quietly affects service intervals and resale mileage.
Questions
205 mm section width, a sidewall 55% of that (112.75 mm), on a 16 inch rim. Overall diameter is 631.9 mm.