How To Read Mountain Bike Tires (Sizes Explained)
Tires are two of the most important parts of a mountain bike, connecting bike and ground. Far from being simply round rubber with knobs, there are many variables to know and consider. Tire width, tread pattern, rubber compounds, and tire casings can be instrumental in how your bike handles. Knowing what is what starts with understanding the meaning behind the labels on any MTB tire.
As an example, I chose a “Maxxis Minion DHR” for two simple reasons: Maxxis mountain bike tires are the most popular and I had one laying around. While some of the acronyms are going to be brand-specific, all tire brands are basically working with the same sort of variables which means that you can apply what you learn here to whatever tire brand you want to use.
In the volume of tire markings, there are a lot of legally required labels mixed with proprietary brand names and marketing trademarks, and even some useless information. With that in mind, how do you read mountain bike tire sizes?
Tire Dimensions
Let’s start with the only standardized and the most important label first: the confusing-looking number salad. There is a lot of information in this short set of numbers and letters. This label is always the same format, using the same units no matter the brand. It’s mandatory for tires to have their dimensions on them, similar to what you know from car tires. As far as labels on an MTB tire go, the dimensions label is the single most important one.
If you ever wondered how you know what size your mountain bike tires are, this label is what you need to find on your tire.
You locate the label, now what do the numbers on your mountain bike tire mean? Since it’s always written in the same format, you can apply these principles to any tire. This is how you read mountain bike tire sizes, regardless of brand.
Outer tire diameter (inches)
Let’s talk about these letters and numbers in the first row: 27.5 x 2.4 WT. I think you already know what 27.5 stands for. It’s the outer tire diameter in inches. That means it’s a 27,5-inch tire in diameter. The standard mountain bike tire diameters are 26″, 27.5″, and 29″.
This is all you really need to know. But in reality, it’s more complicated. That 27.5-inch number – well it’s sort of pulled out of thin air. It’s roughly the outside diameter of the tire when it’s mounted. Of course, the overall diameter is also depending on the rim width and tread pattern. If you want to be really precise about it there’s this other number that you’re going to look at – it’s 584 in my case. It’s written in small letters real close to the bead. Those are 584 mm, which is referring to the bead seat diameter. That’s the literal inside diameter, measured from the tire’s bead.
Tire width (inches)
Let’s look at that next number: 2.5 This is the tire’s width in inches. Which should mean that the tire is 2.5 inches wide, right? Of course not, it’s never that simple. With a profile more like the one of a balloon, what part here is actually measured: the widest part or the part actually having contact with the ground.
2.5″ is roughly how wide the tread width is. For a tire laid flat on the ground, it would be straightforward. But firstly, some brands actually measure this differently. And secondly, it also will vary slightly depending on the rim width.
Similar to the diameter, there’s another number. So, if you go back to that 584 number you’ll see 61. Again in Millimeters, these 61 mm are the literal width of the tire at the widest part when it’s mounted on the correct rim width.
In order to read MTB tire sizes, all you really need to know are the numbers in the first row. There are standardized tire diameter and width, which are the most important and informative measurements of an MTB tire. But if you’re a turbo nerd and really wanna know the real fine details of things, well 584 mm x 61 mm is the tires actual size.
So, what’s the standard mountain bike tire size?
There is no one single standard MTB tire size, however, 27.5″ and 29″ are the most common wheel diameters overall. The standard tire width varies depending on the mountain bike discipline. Cross Country tires are usually the narrowest ones at 2.0″, while Downhill tires tend to be the widest at 2.5″ on average.
Right now the industry trends towards 29″ for Cross Country, Trail / Enduro, and even Downhill wheels. 27.5″ wheels are still very common on Downhill bikes, and lesser so on Enduro bikes. 26″ wheels are not dead yet but are not nearly used as frequently as in the early 2010s when they were the standard. It’s even become quite difficult to find 26″ rims and tires. Some kids’ bikes still come in 26″.
As far as common tire widths go, there are many more viable options. Depending on the type of riding, there are preferable ranges for tire widths.
For Cross Country and Marathon bikes, you’ll want the small profile of a narrow tire. Usually widths from 1.9″ to 2.25″ are used here.
A range from 2.25″ up to 2.5″ is common for Trail, All-Mountain, and Enduro. Here you’ll find many variations in tread patterns, casings, and compounds.
For Downhill 2.5″ is probably the most common width as a compromise between traction and rolling speed, but a range between 2.4″ to 2.6″ is used regularly
These tire measurements are no indication of the actual size of the bike, only the wheels. The bike size is defined by the frame size, not wheel size. Being a 27.5″, a 29″ or a 26″ bike tells you nothing about the bike’s size or geometry. That being said, small frames or kids’ bikes come with smaller wheels as big ones likely don’t fit.
On average, the most common mountain bike tire dimensions currently are 29×2.0 for Cross Country, 29×2.4 for Trail & Enduro, and 27.5×2.5 or 29×2.5 for Downhill (or both for a mixed-wheel “mullet” bike).
A 2.6″ tire width is by far not the end of the road. There are in fact bike tires available at double that width! For more on plus-sized tires, click here. This is the quick version:
Anything from a 2.8″ to a 3.0″ tire width is considered a plus-sized mountain bike tire. These widths sit between the ranges for Trail / Enduro and Fat Bike tires, combining traction and comfort with higher rolling resistance. Since most bike frames don’t fit tires this wide, they are quite uncommon.
But wait! There’s more: Fat bike tires are from 3.7″ up to 5.0″ which is just massive. They look ridiculous for an MTB, more like dirt bike tires than mountain bike ones. Here’s what they are and why you would use them.
Optimal Rim Width (WT)
Right next to the tire size we have two lonely letters: W and T.
What does WT mean on a bike tire?
The label “WT” on an MTB tire tells us the optimal rim width the tire has to be mounted to, which is around 30 mm or wider. This simply means the tire can assume the correct shape and the tire profile, including side knobs, is in the right place.
Mounting a tire marked with WT on a skinny rim results in an incorrect shape and poor (or even dangerous) performance. As is illustrated as “the lightbulb”, this leads to weaker structure and poor, unpredictable handling. The sidewalls sticking this far out are also prone to slices and punctures.
WT actually stands for “wide trail” not “wide tire”, as it has nothing to do with the tire’s width, but the required rim width. This may not make much sense anymore as this indication goes way back to when mountain bikes were using skinny rims instead of the wide purpose-built offroad we are used to now.
So, when smart engineers finally came up with wider rims, other engineers had to come up with tires to match those rims. A wider rim by itself only spreads the tire’s bead apart (refer to “the bell” in the illustration), which creates a flatter top section to the tire. With any of the wrong configurations between tire width and rim width, the tread pattern’s logs are not where they should be for the tire to work as intended and handling, durability and – most importantly – predictability is suffering.
There is more to this discussion. If you want to know more, here is a deep dive into tire sizes.
Model Name (=Tread Pattern)
The tire model name in itself is an informative piece of labeling on an MTB tire. While the branding isn’t standardized or directly comparable, it gives you a clear indication about the tread pattern – the part of the tire actually in contact with the dirt. While the model name is always tied to a specific tread profile, the casings, rubber compounds, and dimensions for that model can still vary.
So, “Minion DHR II” is the model name in my example. It’s one of the most popular Downhill tires used. In this case, the name even includes an acronym suggesting the intended use: DHR stands for Downhill Rear, and DHF for – you guessed it – Downhill Front. Easy enough, right?
There is more to it. While the Minion is indeed designed for gravity mountain biking, it’s not only applicable for full-on downhill bike park use. There are Trail and Enduro versions bearing the same name for those looking for a lighter-weight option.
In fact, the model name of a mountain bike tire says very little about the type of riding it’s best used for. It’s not much more than the name of the tread pattern, responsible for rolling resistance or grip. The durability and handling are dictated by the casing and rubber compounds respectively.
So, while the DHR is a rear-specific tire, you won’t get in trouble for using the DHR on the front and the back. A lot of riders, including me, do exactly that. You will, however, get more flats than you’d like if you use the wrong casing for downhill or riding in rocky terrain. This is why the DHR in the picture is currently on the move from my downhill rig to my enduro bike. I’ll get more into casings later on.
Finding the right tread pattern depends on the kind of terrain but in the end, comes down to personal preference. On the most basic level, tread patterns of MTB tires are designed to strike a specific balance of rolling resistance. The capacity to dig into the dirt is what generates traction by high resistance. Low rolling resistance on the other hand allows tires to roll faster by keeping friction and grip low.
Rolling resistance and grip exist on the same scale of rolling resistance, but are the extremes opposite of each other.
Tread patterns designed for mud riding are arguably the grippiest tires while creating the most rolling resistance with their sharp edges and high knobes. This is a main reason why it requires more energy to ride a mountain bike on asphalt compared to a trekking bike.
Based on rolling resistance and grip there are three major categories to choose from: Cross Country (XC), Trail and Enduro, and Downhill. The distinction is of course not as black and white, but rather overlapping. While DH tires like the Maxxis Minions can make perfect sense on an Enduro bike, I would not suggest putting those wider, grippy tires on for a big XC ride.
As you can see in the graph, XC tread patterns are the fastest rolling but with a low amount of grip. Trail and Enduro are the intermediate tires here, striking a balance between rolling efficiency uphill and traction downhill. And on the far right end, Downhill tires are the burliest providing maximum grip at the cost of efficiency.
With casing options, it’s quite similar as they too are dictated by the intended use like the tread patterns. XC tires are only available in lightweight casing options, while enduro and definitely DH casings need additional protection from higher forces and sharp surfaces. Many of the Enduro and DH tires feature the same tread pattern (model name), but casings and rubber compounds change to suit the riding style and terrain.
Obviously, we are only scratching the surface here. Tire profiles are the bread and butter of mountain biking. This is why I went headfirst into tread patterns here.
PSI Number
P.S.I. in this context isn’t a new acronym specific to the actual make-up or dimensions of an MTB tire. It stands for exactly what you think it would in this context: pound-force per square inch. In other words, psi is a measurement unit for air pressure like “bar”.
So why is it then that Maxxis tells us to run 50 psi in our mountain bike tires? That seems kind of excessive, don’t you think? And you would be right.
The psi number on an MTB tire is by no means a guideline of how much pressure to pump it up to. Rather, it’s indicating industry safety margins and legal requirements. Those psi numbers have nothing to do with the tire pressure that you should be running.
If you look on the side of your tire you’ll see a psi number in multiple places. One is below the dimensions, here it says 50 psi. In another spot, more difficult to make out, it says 35 to 50 psi. Whatever the number is, it doesn’t tell you as a rider anything useful or informative. It just has to be on there legally.
Tire Casing (TPI)
I mentioned casings a couple of times now. What exactly is it and what’s its purpose?
A tire casing is the main rubber body of the tire, not including the tread of the tire. It’s what makes up most of the actual tire, connecting the rim to the tread profile. The tire casing acts as the fundamental support structure of the tire, giving it simultaneously stability and flexibility.
I explain the ins and outs of MTB casings in detail here, but now let’s talk about this 60 or 120 TPI lettering here.
What does 60 or 120 TPI mean on a bike tire?
This label is common across all tire brands and what it’s referring to is the tire’s casing which is a nylon fabric that the rubber is actually molded to.
This cloth is what gives the tire its shape and what TPI means is threads per square inch. In other words: How many threads are in one square inch of the cloth. In the case of this Minion is 120 threads per inch, which sounds like a lot of threads and it is. Some other TPI numbers, that you might see are 60 threads per inch which is half the amount. 60 TPI casings are usually found in less expensive tires. Low TPI makes them heavier and the cloth is stiffer, so they’re less able to conform to the ground at low pressures. That stiffness also comes with increased toughness, so generally speaking, a 60 TPI tire is more puncture resistant.
Here’s the side note: Most tire brands base that TPI number off a single-ply tire casing, meaning it’s just one ply of the cloth. Some others actually fold that cloth over itself to create a dual-ply casing, doubling their claimed TPI number.
Puncture Resistance
In our example case, it’s this little square that says EXO Protection, which refers to the added flat-resistant protection the Maxxis builds into the tire casing when is being manufactured.
Again, all the tire brands are doing something like this and calling it something strong-sounding like: MaxxShield, Kevlar Composite, Silkshield, MaxxProtect for Maxxis, or Raceguard, Double Defense, Dualguard for Schwalbe, or Hardshell, DuraSkin for Continental. You get the idea.
There is no shortage of marketing slang, but the principles are the same. Every tire manufacturer is putting some sort of inserts either in the sidewall of the tire or some sort of casing protection all the way across the top to keep you from tearing holes in your really expensive mountain bike tire by riding through those points rocks.
The flat-protection part is in fact not a part of the casing itself. It’s actually inserted in areas in need of extra protection. That’s done by weaving it in between layers, starting down at the tire’s bead and going up to just below the side knobs.
Manufacturers put protection in the sidewalls because when you’re riding your tires squishes together, pushing the sidewall further out, making it prone to hit rocks and roots on the trail. That’s the EXO part, colored green in the picture.
As you can see, it doesn’t protect the top of the tire, only the sidewalls. For additional protection you would need on hard-hitting downhill tires, there’s more armor available. In our case that would be EXO+, which comes in with the regular EXO layer in addition to their “silk shield” technology.
This thinner protective layer runs from the bead on one sidewall all the way up, under the tread, and down to the bead on the other side. Again, that’s another layer woven into the tire casing combined with the sidewall protection.
So while one layer of protection (EXO) only protects the sidewalls, another casing insert (silk shield) covers the entire casing bead to bead to give provide better protection along the top of the tire.
Rubber Compound
How do you know what compound your tires are? This label informs you of what sort of compound construction is used to create the tread (or profile) of the tire. In our Maxxis case, it’s this little three with a big C next to it.
The number 3 refers to the amount of three and the C to compound, so that means this tire is made using three different rubber compounds of varying firmness. This is a common technique in the tire industry. Pretty much every reputable tire brand is combining different types of rubber together to create tires of different characteristics for weight, grip, durability, and reliability.
And every single manufacturer uses their own naming scheme, which can get quite confusing. That’s why I picked out the most popular brands to explain their compounds in their own articles. For more information, check out the explanations for Maxxis MTB tire rubber compounds and Schwalbe Addix MTB tire rubber compounds.
For a complete list of all the MTB rubber compounds of the major MTB tire brands right now, check out this article.
Accordingly, there are a couple of tire compound numbers varying in complexity. For mountain bike tires you can choose between single compounds (1C), dual compounds (2C), and triple compounds (3C). There is in fact even a quadruple compound (4C), that is currently only made by Vittoria.
Let’s stick to 3C for now. This means there are three compounds, but not which ones. There are different kinds of 3C or triple compounds, depending on the firmness of the rubbers used. This particular one is labeled “MaxxTerra“, which is a name branded by Maxxis. There’s also “MaxxGrip” and “MaxxSpeed”. Most brands have their own nomenclature for their compound variations, so it’s a little annoying to navigate through, but in the end, they basically all name the same things differently.
The tire compounds all fall on a spectrum between grip and durability – from soft to firm.
To stick with our example, MaxxTerra basically means that Maxxis put firm rubber on the bottom for support, medium rubber along the middle (that provides traction but also doesn’t wear out so fast), and then some softer, slower rebounding rubber on the side (that’s where your cornering grip comes from).
The idea here is to put the grippy compound where it meets the ground, the longer-lasting compound in the middle, and then the hard compound at the bottom for support.
There’s also MaxxSpeed and MaxxGrip, which are named appropriately. With those, you can pretty much figure out what they’re doing by looking at the names. For MaxxSpeed that would be faster rolling so it has overall firmer, durable rubber compounds but it still has soft rubber on the side knobs. And MaxxGrip is all about traction by being softer overall. It’s what you want if you don’t care how long your tire lasts.
Tubeless Capability
And finally, we have this big TR logo right here and below it says tubeless-ready. So that kind of gives the meaning away. There’s a similarly telling name every brand uses (TLR, TLE and so on), so you’ll easily spot it.
As with most labels, this one too tells us more than you would expect at first glance. TR informs you about two different things:
First, the set of rubber compounds used for the tire is compatible with some of the nasty stuff that’s in some tire sealants out there. So, this isn’t going to degrade the rubber, that the tire’s made from.
Secondly, it tells us that the tire’s bead (the part pushing against the rim) is shaped in a way so that it locks securely to the rim. This is necessary for the tire and rim to actually be airtight and securely lock the tire in place, especially under heavy braking. You would not want to have the tire rotating, while the rim is not.
These two factors are why you want tubeless-ready tires instead of trying to make your non-tubeless tires tubeless. It won’t work and can even result in a nasty crash when the tire leaves the rim. Don’t ask why I know …
Rotation Direction
Often a bit hidden and out of view is another critical marking. As the name suggests it indicates the direction the tire is supposed to be mounted in order to rotate in the correct orientation. This is important because an MTB tire is supposed to offer the best rolling resistance while riding, with the best braking performance for any given profile.
Having it face the wrong direction will net you the worst of both worlds: high rolling resistance with weak resistance under braking. For more details on tread patterns, I would refer you back up to the corresponding passages.
On most tires, you can actually clearly see based on the tread pattern how it’s supposed to roll and interact with the ground. In any case, you got the label to fall back on.
