The size label on a tyre indicates the tyre width, its sidewall height as a percentage of its width and finally the wheel rim size it is designed for. As an example a 205/50R16 tyre is 205mm wide, an aspect ratio of 50% which would be 102.5mm and would be suitable for a 16-inch wheel.

Increasing the aspect ratio to 55 would give a sidewall of 112.75mm, an increase of 10.75mm. At an indicated 60mph this would give an actual speed of 62mph, and at an indicated 100mph this would give an actual speed of 103mph.