According to the American Motorcycle Association, motorcycle tire diameter can be determined by looking at the numbers stamped on the tire; the last two numbers provide the diameter in inches. Measurements are either marked in metric designations or using an alphanumeric code.
The American Motorcycle Association explains that the metric designations provide the tire's width and the aspect ratio between the width and height in millimeters, as well as the tire's diameter in inches. The alphanumeric designation breaks down into the usage (M for motorcycle) the size code (T for 110mm) and the aspect ratio in millimeters, followed by the diameter in inches. The measurements 110 / 90 15 and MN90 15 both indicate a tire of the same size, with a 15 inch diameter.