Compare two tyre sizes: diameter difference and speedometer error.
The first number is the width in millimetres, the second the sidewall height as a percentage of that width — 55% of 205, or 112.75 mm. R means radial construction, and the last number is the rim diameter in inches. The overall diameter follows from those three.
The accepted limit is three percent of the original diameter. More than that shifts the speedometer, affects ABS and stability systems, and can let the tyre rub the arch. This calculator shows the difference as a percentage so it can be checked before buying.
A speedometer counts wheel revolutions, not distance, so a larger diameter makes it read lower than your true speed. Two percent more diameter at an indicated 90 km/h means an actual 91.8 — and that gap is what earns a fine.
Winter tyres are compulsory from 1 December to 1 March, and the tread must be at least four millimetres deep during that period. In practice people fit them once the daily average drops steadily below seven degrees, since summer rubber hardens in the cold.