View Here : Tire Size Meaning
ATV Tire Size Format. Tire Width is the width of the tire, measured in millimeters, when the tire is mounted and inflated to the recommended pressure. The Aspect Ratio is the tire section height divided by the section width (not tread width), multiplied by 100. The section height is measured at the center of the tire crown.
255 - This is the tire's width in millimeters. 45 - This is the tire's sidewall height, expressed as a percentage of the width figure, also known as an aspect ratio. R - This is the tire's type of construction, and is almost always an "R". 15 - This is the wheel diameter the tire is designed for, listed in inches.