What Does 315/40R21 Actually Mean?

Let's break it down simply: The 315 means this tire has a tread width of 315 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 21 indicates that this tire perfectly fits a 21-inch wheel.