The first number or second letter in a tire size represents the nominal width. Width is measured in a straight line from the furthest point on one sidewall, across the tread, to the furthest point on the opposite sidewall. If there is any question whether or not a larger than OEM tire will fit your bike, you're encouraged to call Tech Service. The different size numbering systems specify widths in different measurements. You can view a width cross reference table below

You use the aspect ratio to indicate a tire’s cross-sectional profile. A smaller number means a lower profile, and the height-to-width ratio is shown as a percent. For example, if the aspect ratio is 90, that means that your tire’s cross-sectional height is 90% of its width. If the tire has the metric, alpha or low-profile inch numbering system, the aspect ratio will appear immediately after the width

Rim diameter is the diameter of the rim/wheel on which the tire will be mounted, in inches.