An alphanumeric designation such as MT90 17, represents the same tire size as 130/90 17. It's just expressed in a different way. M means the tire is for motorcycle use, T is the tire width code, 90 is the aspect ratio and 17 is the rim diameter.
What size is a MT90 tire?
Metric Alpha Inch
------ ----- ------------
100/90 MM90 3.25 to 3.50
110/90 MN90 3.75 to 4.00
120/90 MR90 4.25 to 4.50
130/90 MT90 5.00 to 5.10
These digits after the tire size, 90S in this example, are the "load index" and "speed rating". The numbers are what is known as a load index, and specify what the maximum weight capacity of the tire is.
Permissible Rim Widths Metric Standard Inch
---------------------- ------ -------------
1.85, 2.15 90 3.50
2.15, 2.50 100 3.75
2.15, 2.50, 2.75 110 4.00
2.15, 2.50, 2.75 110 4.25
Does the tire aspect ratio matter?
Aspect ratio is an important sizing calculation in tire fitting, and should be considered with wheel diameter with the best tire and wheel combinations. Lower aspect ratio typically indicates a high performance tire, with better lateral stability.
How does aspect ratio affect motorcycle handling?
For sportbike/racing tires, the aspect ratio helps to determine the "turn-in" characteristics of the tire. If it has too low/wide of a profile, it is very hard to turn into a corner (hard to force the bike to lean). If it is too tall, the bike is very "skittish" at turn-in, and can be downright unstable and scary.
Are wider tires better on a motorcycle?
Narrower, lighter tires are definitely better for performance and handling, but they can't handle as much horsepower. If you enlarge the contact patch by making the tire wider, you get more grip and that larger contact patch dissipates more heat and provides more material to wear away, so the tire lasts longer.