What Does 305/40R21 Actually Mean?

Let's break it down simply: The 305 means this tire has a tread width of 305 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.