Two coding systems are used to size motorcycle tyres. One uses numerals to express the tire size; the other uses a combination of numerals and letters. Either system, however, may add letters to the size to give other information about a tire.
Where to look
Look on the sidewall for the tyre size. Examples of tire sizes are 130/90 16 and MT90 16.
All tyre sizes end with the diameter, in inches, of the rim that goes with that tyre. Tyres sized 130/90 16 and MT90 16 would both fit a 40 cm (16 inch) rim.
The tyre's height divided by its width, or "aspect ratio," is the number preceding the rim size in both tire sizing systems. On a 130/90 16 and a MT90 16 tyre, the aspect ratio is 90, meaning the height of the tire is 90 per cent of its width.
An all-numeral tyre size such as 130/90 16 means the tire is 130 mm (5 inches) wide on a 40 cm (16 inch) rim with an aspect ratio of 90. This method is called "metric," even though the rim size is in inches. Some tyres show width in inches and without the aspect ratio -- for example, 5.00---16 for a tire 13 cm (5 inches) wide on a 40 cm (16 inch) rim (with the dash showing the tyre is bias-ply construction).
Numbers and letters
In the older "alphanumeric" sizing system, a letter to the left of the aspect ratio indicates tyre width. For example, on a tyre sized MT90 16, the T means the tire is 130 mm (5 inches) wide. The second letter is the one that matters -- they all start with M, for "motorcycle."
Remember, letters to the left of the aspect ratio show tyre width in the alphanumeric sizing system. Letters that show up elsewhere in either tyre sizing system show speed rating and construction of the tyre; for example, R or B means the tire is radial or belted-bias construction, and Q, S, T, U, H, V, W, Y and Z indicate maximum speeds the tyre can handle.