What Does 320/55R18 Actually Mean?

Let's break it down simply: The 320 means this tire has a tread width of 320 millimeters. The 55 is the aspect ratio, which means the sidewall height is 55% of that width. The "R" stands for Radial construction (the modern standard for safety and durability), and finally, the 18 indicates that this tire perfectly fits a 18-inch wheel.