Tyre Aspect Ratio

The aspect ratio of a tyre is the height of its sidewall expressed as a percentage of the tyre's width. In the size 180/55 ZR17, the tyre is 180 mm wide and the sidewall is 55% of that, about 99 mm tall. The last number is the rim diameter in inches.

A lower aspect ratio means a shorter, stiffer sidewall and usually sharper steering. A higher ratio gives a taller profile, which changes ride height and how quickly the bike tips into a corner. On motorcycles it also affects how the tyre profile matches the rim.

Fitting a different aspect ratio than the manufacturer specifies changes geometry, gearing and speedometer reading, and may not be road-legal. Stick to sizes listed for your bike unless you know exactly why you are changing.