BIKE TIRE SIZE GUIDE
October 13, 2021
Your tires are your only physical connection to the ground when you’re riding your bike. It’s a bit wild when you really think about it, but every time you’re railing a berm, picking your way through a tangle of roots, white-knuckling a loose gravel descent, or doing wheelies in the parking lot, there are just a few square inches of rubber (your “contact patch”) that are doing all the work at any given time.
It’s enough to make you realize how important tire choice is to your performance and safety. Picking the perfect tire starts with finding the right size.
HOW TIRE SIZING WORKS
Tire sizes are typically labeled as the outer diameter followed by the width (29 x 2.3" or 700c x 38 mm, for example). Note that most tire makers label mountain bike tires in inches, and road and gravel tires in millimeters. Let’s get into diameter and width:
We admit that this gets a little confusing, so hang on.
- The 29 in a 29" mountain bike tire indicates the outer diameter of the tire when it’s mounted on the appropriate size wheel.
- 29" tires are designed to pair with 29" rims
- 29" rims only measure about 24.5" (622 mm) in diameter, measured from where the bead of the tire sits when installed—this is known as the “bead seat diameter”. Remember that the 29" only refers to the diameter of the entire wheel with a tire installed and inflated
- Note that the wheel and tire will rarely measure out to exactly 29" due to varying tire widths, cross-sections, and casing volumes
- All of these principles also apply to 27.5" wheels and tires (which use a 584 mm diameter rim)
Road and Gravel Tires
- The 700 in a 700c road or gravel tire also indicates the outer diameter of the rim and tire combination, but in millimeters.
- 700c tires are designed to pair with 700c rims, which measure to the same 622 mm in diameter as 29" rims
- As with 29" wheels, there are so many 700c tire widths that the actual installed diameter will rarely measure out to 700 mm.
- All of these principles also apply to 650b wheels and tires (which use a 584 mm diameter rim)
You’re in luck: this part is much simpler. Tires are labeled with the width that they are designed to be when they’re properly inflated on the correct-size rim. For example, a 29 x 2.3" mountain bike tire will measure 2.3" at its widest point, and a 700c x 47 mm gravel tire will measure 47 mm.
In addition to a tire’s labeled size, you’ll also find an ISO size molded into the sidewall. The ISO system uses two numbers. The first is the designed width of the inflated tire in millimeters. The second number is the bead seat diameter in millimeters.
For example, the ISO size of a 29 x 2.3" mountain tire would be 58-622 and a 650 x 40 mm gravel tire would be 40-584.
CHOOSING THE RIGHT TIRE SIZE
For the best performance, you’ll want to choose a tire size that’s compatible with your frame, fork, and rims. Running the wrong size tires can hurt ride quality, damage your rim or bike, and can be a safety hazard, so it’s important to confirm sizing before purchasing new tires.
Road and gravel tires typically come in two diameters: 700c and 650b. Width can vary from around 30 mm for a light and fast gravel race tire to around 47 mm for stability and traction on chunky gravel. If you’re replacing your tires, you can simply look at the size printed on their sidewalls and purchase that size. But if you’re looking to switch to a different tire width, you’ll want to pay attention to your frame clearance and rim’s internal width.
Also note that the closer you match the tire to its intended internal rim width, the more accurate the tire’s labeled size will be—no one likes to find out that their new 2.4" tires only measure out to 2.2".
There should be at least 6mm between the outer edge of your tire and your frame or fork to avoid rubbing.
RIM INTERNAL WIDTH
As a general rule, your rim’s internal width should be 40–80% of your tire’s width, however your rim manufacturer may recommend a different range for optimal performance. If you are unsure about your rim’s internal width, consult the rim manufacturer.
Now you can navigate the world of tire sizes and find the rubber that will work for your bike. Consult your local bike shop with any further questions, or contact us with questions about Teravail products.