What Does 125/40R18 Actually Mean?

Let's break it down simply: The 125 means this tire has a tread width of 125 millimeters. The 40 is the aspect ratio, which means the sidewall height is 40% 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.