BIKE TIRE SIZE GUIDE
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 connecting rubber to road.
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 and the right tread.
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:
DIAMETER
We admit that this gets a little confusing, so hang on.
Mountain Tires
- 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)
32 Inch Tires
- While 32 inches sounds like a big jump from 29” or 700c, the reality is a bit more complicated due to the math of circumference and diameter.
- The rims measure 682mm which is indeed a 60mm jump from traditional 700c/29” rims.
- 60mm is 2.36 inches, so the actual wheel / tire bead size increase is fairly proportional to the increase in tire diameter.
- The benefits of 32” bike tires are most apparent in situations where more rollover and rolling speed are desirable.
- 32” tires are an exciting new realm in cycling, with new products constantly emerging to give riders more choices for dialing in their bikes.
WIDTH
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.
ISO SIZING
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, which is the same as the size of the rim you are mounting the tire to.
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.
WHEELS GIVE TIRES THEIR SHAPE
If the tire contact patch is the most important part of connecting your bicycle to the road, then the wheels your tires are mounted on are arguably the second-most important part of the system. Hubs and spokes influence how your bike rolls, absorbs bumps, and respond to pedal input.
Rims provide the right stiffness-to-compliance ratio and feature application-specific width and shape to help your tires measure out to their intended size and shape.
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 55 mm for stability and traction on chunky gravel. In today’s world, many gravel bikes even fit what were once considered mountain bike tires, like 29 x 2.1” cross-country treads. 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,” which can happen if they are mounted on especially narrow rims.
WHY CHANGE SIZES
Within respective disciplines like mountain or gravel riding, there are a number of reasons you might pick a wider or narrower tire. Generally speaking, a narrower tire offers lighter weight and lower rolling resistance, while a wider tire focuses more on outright grip. Even subtle tweaks can help to refine your bike’s riding characteristics a lot! Experimenting with tire size and tread pattern is one of the most impactful ways you can alter your ride.
MIX AND MATCH
It is common practice in the mountain bike world to ride a wider, more aggressive tire on the front with a narrower, faster-rolling tread on the rear. This setup is increasingly finding traction in the gravel and adventure cycling worlds as people recognize the potential for a more fine-tuned ride. The front tire is responsible for steering, which is why riders gravitate towards more grip, while the rear can be said to “follow” the front, which is why many riders elect to pick a somewhat lighter and faster-rolling tire to gain some speed and efficiency at the expense of outright rear tire grip.
It is important to know your terrain, riding style, and goals when selecting a tire combination.
Clifty Tire
Ehline Tire
Kessel Tire
FRAME CLEARANCE
There should be at least 6mm between the outer edge of your tire and your frame or fork to avoid rubbing. If you often ride in extremely muddy conditions, then it is extra-important to factor in additional clearance between tire and frame to avoid frame damage or getting so caked in mud that your bike no longer rolls.
Most bike frames have published maximum tire size information, though if you are getting more experimental, a good set of measuring calipers are a must!
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.
TREAD PATTERN
For gravel bike tires and mountain bike tires, tread pattern choice is influenced by the terrain you ride most often. Picking tread pattern is often a mix of art and science, as everyone has preferred riding characteristics. The general rule is that a knobbier tread pattern provides more grip in a wider variety of settings, while a smoother tread is faster rolling but less grippy.
TIRE SHAPE
Bet you didn’t realize that tires could have different shapes! While all of tires are round, the shape of their tread blocks varies with intended purpose. A super knobby mountain bike tire will often feature what is known as a “square” shoulder lug pattern, while gravel tires can be more squared off for grip in sketchy conditions or rounder and smoother for fast and predictable performance in dry conditions where outright speed is most important.
FINDING THE RIGHT BALANCE
Many tires are designed to strike the right balance of fast-rolling center treads and more confident shoulder lugs to give you some extra grip in sketchy corners and loose descents.
CASING AND COMPOUND
Similar to selecting width and tread to match conditions, many tires are made in multiple casings or rubber compounds to offer more ability to fine-tune your ride characteristics to match the terrain you ride.
Going for PRs or podiums on a smooth and dry course? Light and fast tires are your friend.
Frequently find yourself in chunk and unpredictably rocky terrain? More durable tires are worth the extra weight every time.
TIRE WEIGHT
The weight of your tire and wheel system makes a bigger difference on overall ride experience than saving weight on “static” components like bottle cages or seatposts. That’s because rolling mass has inertia that impacts how quickly it can change direction – which means steering, acceleration, and braking are all impacted by weight. As with other aspects of tire selection, you have to make the right choice for your goals and terrain. Lighter tires typically sacrifice outright durability for racier and more supple ride feel, while more durable tires add mass in order to provide superior puncture protection and more grip through heavier lugged designs.
BUILD A SYSTEM THAT WORKS FOR YOU
Choosing the right wheels, tire size, and tread pattern might sound like a daunting task, but it’s really simple! The size of tires you can run is largely influenced by the frame you are mounting them to and the conditions you plan to ride in.
The more you know about the main differences in tire design, the easier it is to make the right choice for you. Still unsure?
Ask our experts which tires are right for you.
Or just experiment – that’s part of the fun of riding a bike!
