You may have noticed these numbers that appear along with the tire’s traditional size. We’ve been printing them on our tires for more than a decade. GMS is the most accurate way to determine a tire’s true size. It is comprised of two numbers: The first number is the tire’s width—in millimeters—at the casing. The second number is the width at the tread. GMS can reveal a lot about size, shape and handling characteristics. Take the WeirWolf LT for example: this 2.55-inch tire has a GMS measurement of 58/55mm. These numbers tell us that the WeirWolf LT is actually wider at the casing than at the tread, meaning that this tire is “round” in profile. Because of these fast-rolling, low profile knobs, the WeirWolf LT rolls with impressive efficiency. And the massive casing absorbs trail chatter providing a smooth ride. But this tire’s low-tread (that’s what the LT stands for) and round profile also mean that the WeirWolf LT doesn’t penetrate loose soil like a square profile tire (the Timberwolf 2.5, for example). The Timberwolf 2.5 sits at the opposite end of the spectrum with a GMS measurement of 55/65. The Timberwolf is a whopping 10-millimeters wider than the WeirWolf LT at the tread and actually more narrow at the casing!

You can also use GMS to address which tires will fit which bikes. An older generation XC bike will likely have limited space for anything wider than a 2.2-inch tire. If you’re uncertain of just how much space you have, don’t be afraid to pull out a ruler and measure the gap between your chainstays. If you have a total of 65-millimeters of clearance, your maximum tire width is really around 58-millimeters (you will need a little extra space to compensate for wheel/frame flex, soil stuck to your tires, etc.). There’s really no black magic at work, just a ruler and commonsense. And, best of all, GMS eliminates the guesswork.