When looking at a tire size, the first number represents the nominal width and is usually represented in millimeters. This measurement represents the distance between the outermost edges of each sidewall.
This number indicates the aspect ratio. This ratio is used to determine the height of the tire’s cross-sectional profile. Expressed as a percentage, an aspect ratio of 55 means that the cross-section of the tire is 55% as high as it is wide.
Measured in inches, this number represents the diameter of the rim on which the tire will fit.