Tire size comparison
Enter two tire sizes (e.g. 205/55 R16 and 225/45 R17): calculate diameters, circumference and percentage difference.
What it's for
Changing tire size changes the wheel's overall diameter: this affects the speed shown by the speedometer and the odometer's km. Here you see the percentage difference.
Rule of thumb
To avoid throwing off the speedometer, the diameter difference should stay within ±2-3%. A larger tire makes the speedometer read below the real speed.
What 205/55 R16 actually means
The three numbers say different things: 205 is the width in millimetres, 55 is the sidewall height as a percentage of the width (so 112.75 mm) and 16 is the rim diameter in inches. The overall wheel diameter comes from the two sidewalls plus the rim.
It is the 55 that misleads: it is not a measurement, it is a ratio. Going from 205/55 to 225/55 does not only widen the tyre, it also raises the sidewall by 11 mm on each side, because 55% of 225 is more than 55% of 205.
Why the speedometer lies
A speedometer counts wheel revolutions, not metres: the control unit multiplies by the factory circumference. Fit a larger diameter tyre and each revolution covers more road, so your real speed is higher than the one shown; a smaller one does the opposite.
The same error lands on the odometer, and therefore on service intervals, quoted consumption and resale value. By law a speedometer may read above the true speed, never below: which is why manufacturers already set it a few percent optimistic.
Equivalent sizes and their limits
Staying within ±2-3% of diameter is the practical rule, but it is not the only constraint: the size must be one of those listed on the vehicle document or approved for that car, otherwise the inspection fails and after a crash the insurer has an argument.
The load and speed ratings, the two symbols at the end (91V, for example), must be respected too. A load index below the prescribed one is dangerous when fully laden, and a lower speed rating has to be declared: they are numbers, not suggestions.
Nearby tools
The full meaning of the code stamped on the sidewall is explained by Read the tyre code. To see how much tyres matter when you need to stop, look at Braking distance and Safe following distance.