Mountain bike tires come in only a few relevant sizes. While there are only two diameters (29 and 27.5 also known as 650B) for adult riders, tire width is where the fine-tuning is at.
Every mountain biking discipline has preferred tire widths. The diameters are currently pretty much the same for all kinds of mountain bikes with 29-inch wheels being the most widespread.
Key Takeaways
Mountain bike tires are standardized and available in diameters of 27.5 or 29 inches and widths between 2.2″ and 2.6″ generally.
29×2.4 is the most common size currently (see table below). 27.5″ rear tires are common for smaller riders.
Bigger tires roll over obstacles better but don’t handle as well as smaller tires. And with modern mtb tire casings, wider tires offer good rolling resistance with better grip at little cost to rolling resistance.
Still, narrow 2.2″ tires are king for mud tires.
All in all, tire treads and tire pressures have a similar impact on riding as wheel size (source).
While wheel size has performance implications, in my experience it’s also very much about personal preference and trust in the tire.
Before we get into the pros and cons of various sizes, we gotta cover the basics.
Reading size labels
Luckily for us, MTB tire sizes are very standardized. So size labels are very easy to read. Here’s an example:
27.5X2.40
27.5 refers to the outer diameter in inches
2.40 refers to the tire width in inches
(WT just means Wide Tire, which is useless information for MTB since all are wide)
Width and diameter are all you need to know.
Tire manufacturers have to adhere to the European Tire and Rim Technical Organization (ETRTO) so every brand uses these same standard measurements.
Related: how to read MTB tire labels
Most common MTB tire sizes
What tire dimensions you can use depends on the bike frame and the wheel size (rim diameter and rim width). Mountain bikes are designed for specific wheel diameters and tire widths.
So, the manufacturers already pick the best dimensions for each bike based on the discipline and type of riding it’s designed for.
As you can see, modern mountain bikes basically only vary in tire widths:
| MTB Discipline | Diameter | Tire Width |
|---|---|---|
| Cross Country | 29″ | 2.2″ – 2.4″ |
| Trail | 27.5″ & 29″ | 2.3″ – 2.4″ |
| Enduro | 27.5″ & 29″ | 2.3″ – 2.5″ |
| Downhill | 27.5″ & 29″ | 2.4″ – 2.6″ |
| Fatbike | 26″ | > 3.0″ |
Surprisingly, XC and DH tires are pretty much the same size now as a 2.4″ width has become the norm with XC racers. This is what I noticed when walking through the World Cup pits in Leogang (where XC, DH and EDR raced).
On a given bike you can’t just switch from a 27.5 to a 29er and vice versa. The wheel diameter and also the tire width (to a certain extent) are dictated by the bike frame.
Plus-sized mtb tires (27.5+) are more a thing of the past since 29″ wheels became the norm across most MTBs.
Picking the right tire width (for performance)
Choosing optimal tire size is often only a choice of tire width since diameters are so standardized over all disciplines. We’ll cover 27.5 vs 29 further down.
Only on Enduro and DH bikes do you even still have the option to choose between 29, 27.5 and mullet configurations. That’s because you often swing of the rear of the bike and may buzz the tire.
29ers are for tall riders and racers, mixed-wheel bikes (mullets) are for racers and park rats and good old 27.5ers have become pretty rare but are preferred by smaller riders or riders who want a more playful bike to flick around.
The regular MTB rim with a 30mm internal width can accommodate a wide range of tire widths from 2.2″ to 2.6″. So feel free to experiment.
Is wider better?
Bigger wheels in overall diameter generally roll faster. This is due to higher pedal efficiency, meaning more distance is traveled with the same power input at the pedals. They also roll more steadily over obstacles preserving forward momentum.
So bigger, wider tires provide more traction and trust for a minimal cost of weight and handling.
However they do not turn as well and can downright make tight turns clumsy. But on most cross country tracks and big downhill lines with wide turns, they feel right at home.
When I switched the front tire on my downhill bike from a 2.6 I’ve been riding all season to a 2.4 it was a big change. 2.6 felt like a monster truck while 2.4 felt twitchy and nimble in direct comparison. And that’s just a 0.2-inch difference!
I wouldn’t say one is better than the other – there is no right or wrong – but there are clear benefits and costs associated.
Tire Width Explained
It used to be that narrow tires had less rolling resistance. But with modern mtb tire casings, they hold their shape pretty well. That way we can use lower tire pressures and wider tires for added grip and increased trust with no downsides.
Wider MTB tires (2.4″-2.6″) provide a larger contact surface and an aggressive tread pattern (since they’re used in DH and Enduro). Which provides maximum traction and stability on most terrain.
Narrow tires (1.9″-2.2″) are the exact opposite with a minimal contact patch and smaller tread patterns (since they’re used in Cross Country). They are lighter, are more nimble in handling and can dig into softer dirt or mud.
That’s why full-on mud tires are generally narrower.
Often front tires are wider than rear tires to cater to what each tire is doing. While the front initiates turns and has to grip reliably, the rear (where most of the weight is) needs to roll fast.
Fat bike tires on the far extreme take full advantage of what maximum tire width can offer (comfort and grip), but at the cost of increased negative side effects like high rolling resistance, sloppy handling and bounce.
27.5 vs 29
Wheel size is a fundational decision between different bike designs, rather than simply wheel sizes. This is because the frame and suspension fork have to be sized in accordance with wheel clearance.
Note: This has nothing to do with frame size! Although smaller frames often can’t fit big 29ers
These are general advantages and disadvantages:
| 27.5″ | 29″ | |
|---|---|---|
| Energy Expenditure | Less mechanically efficient to pedal | Lower heart rate and caloric expendutire |
| Grip | Good balanced grip | Larger contact patch (more traction) for the same tire width |
| Handling | Playful and agile handling | Stable, but less maneuverable on tight trails |
| Speed | slightly slower in terms of top-end speed | more efficient at rolling over obstacles and providing momentum |
| Suspension Travel | Suspension can have more travel with smaller wheels | Less clearance between butt and rear tire |
| Weight | Lighter | Larger size means 29ers are heavier |
| Comfort | More feedback on rough trails | Smoother ride quality and better momentum |
So, the choice between 27.5 and 29 is made with the bike purchase. It’s set in stone after that.
A comparison of 26-inch and 29-inch wheel sizes revealed that 29ers led to faster speeds, lower average heart rates, and lower total caloric expenditure over similar trail rides.
Impact of Wheel Size on Energy Expenditure during Mountain Bike Trail Riding
First, you select for the discipline & suspension travel and then (if more wheel sizes are even available) between 27.5″ (also known as 650B) and 29″ wheels.
Or both on mixed wheel “mullet” bikes which have a 29er front tire and 27.5 rear tire for the best of both worlds.
There is no switching between the two on most bikes. But you can still play around with tire width to change rolling resistance, grip, acceleration and comfort.
Turning 27.5 into 29ers using wider tires
Here’s the kicker: If you measure your tire, chances are it isn’t exactly 27.5″ or 29″ tall. Of course not. Since you can put such a wide range of tire widths on the standard 584x30mm rim, the overall dimensions can also change.
More tire volume = larger tire
So 27 and 29 are not exact measurements, but rather names for standardized rim and tire sizes.
Putting a wider tire on a given rim increases the overall wheel size, while a narrow tire makes it smaller.
Here are 4 different tire dimensions side by side. Notice the diameter changing with the tire width.
The big 27.5×2.8 is pretty much the same diameter as the 29×2.3. And quite a bit wider, which will rub on some bikes’ rear triangles and as such is not recommended without any constraint.
This is just to illustrate that it’s not all about just 27 vs 29 but also a question about the width of tire you put on the rim.
