Choosing the tire size, tread, and tire pressure affects your comfort and the capabilities of your gravel bike and bikepacking rig.
Riding unpaved roads complicates tire selection. Riding on pavement is fairly predictable. But once you roll off the paved road, no two tours or gravel rides will be the same. There is a huge difference between a leisurely day ride, a competitive "gravel grinder" and an extended bikepacking trip.
"Gravel" can mean anything from hardpacked, pristine crushed red granite here in Central Wisconsin, to chunky broken rock and sand in Northern Wisconsin, to peanut butter mud in farm country. “Rail trails” and many Midwest gravel roads are nearly as smooth as paved country roads. So are desert roads - that is, until it rains and they become impassible!
We love the adventure and challenges of gravel cycling - but the right tires are crucial!
The previous article, "What is a Gravel Bike?" covered the MANY reasons why cyclists are rediscovering the joy of unpaved roads.
But we don’t want to flounder because we were unprepared. In the words of Selene Yeager, author of Gravel, The Ultimate Guide, “Gravel remains largely undefined, which is exactly the point. It’s supposed to be an adventure.” She continues, “TIRES! Forget about the bike! Gravel bikes, by nature, are designed with primarily one thing in mind: being able to fit big, ground-gripping tires in the frame while still delivering a fun, functional ride. It’s impossible to overstate the difference the right tires and tire pressure can make on your overall gravel riding experience.”
The best gravel bike and rider are rendered helpless with the wrong tires.
Your bikepacking trip is miserable (or impossible) with the wrong tires.
How do you sort out the many tire and wheel options available? Scan the specs of gravel bikes, touring bikes, and MTBs and you will be bombarded with a confusing array of inches and millimeters, plus designations with “C” and “B” added to rims and tires.
First, let’s look at the rims that are typically found on gravel bikes.
Gravel bikes fit between road bikes and mountain bikes, with some overlap on both ends of the range. In his book, Gravel Cycling, Nick Legan tries to sort out the options. He writes, “When describing a rim, road bike companies will describe it in metric terms, while mountain bikers stick to imperial measurements. This is maddening, especially when they are describing the exact same dimension.”
For example: 700c = 29 inches
"700c" is by far the most common size on road bikes, cyclocross bikes and gravel bikes. Using the same diameter rim, mountain bike call this a “29er”, the approximate overall diameter with a 2-inch tire.
All bike frames are narrower at the apex of the rear triangle and fork crown. Many road bikes are limited to 32mm tires – which is also the limit imposed by ICI, the governing body for cyclocross racing. In contrast, many gravel bikes can fit tires from 40mm up to 50mm on 700c wheels.
Many gravel bikes can accommodate wider tires by interchanging smaller diameter wheels. 650b = 27.5 inches
650b (27.5") is the middle ground between 29” and the traditional 26” wheels whiich were originally fitted on mountain bikes. Gravel riders tackling single track trails have adopted this the rim size to allow for tires from 40 to 55 millimeters, or wider. Heavily loaded bikepackers and round-the-world bike tourists opt for 27.5" wheels for the larger tire cross-section with greater air volume, which provides more weight capacity and a smoother ride.
The smaller diameter 650b rims provide the room for larger tires on the same frame.
The bike that can fit 700c x 40mm tires may have clearance for 650b x 50mm tires, or larger. Many riders mistakenly assume bigger is better. They assume that 700c wheels are ALWAYS the fastest wheels on and off road (or 29ers, in the case of mountain bikes.) However, research has shown that tire width doesn't have much impact on speed or rolling resistance.
You will find that 700c x 38mm tires have basically the same diameter as 650b x 47mm tires. Since the gear ratio of your bicycle factors in the tire diameter, these two wheel sets share the same gearing, as well identical bottom bracket height for handling and clearance, along with identical toe clearance when steering.
The smaller 650b wheelset might actually weigh less than a 700c set AND be stronger!
Which tires and wheels are BEST for gravel bikes or bikepacking?
The answers depend on many variables, but the two most important factors are the total weight of the rider and bike, then the road/trail conditions.
First, let’s discuss rider plus bicycle weight. The heavier the cyclist, bike, and gear the higher the tire pressure that is required. Higher pressure provides a rock hard tire that rolls with less effort but is also tiresome as it transmits more road shock and vibration. Lower pressure offers a softer ride on rocks and gravel. But lower pressures also increase the likelihood of “pinch flats” when the inner tube is cut by the rim on rough trails. Add 20 or 30 pounds of bikepacking gear and you will need higher tire pressures.
For example, a 110 pound woman may need only 50 PSI while a man with gear totaling 200 pounds may require 75 psi while riding the same rims and tires on the same gravel course. That is 50% higher air pressure for the same the tires on the same course.
Let's pause to discuss TUBELESS tires for gravel bikes and bikepacking.
Selene Yeager doesn’t hesitate when she says, “There’s one thing all the experts agree on 100 percent: Go tubeless. No matter what size and tread you choose, make sure they are tubeless. The ability to run lower pressures will dramatically improve your ride experience.”
Modern tubeless tires really have NO downside for road bikes, gravel bikes, and bikepacking.*
The ability to ride with lower pressure will result in a more comfortable ride with better traction.
By removing the inner tube, you also benefit from a significant reduction in wheel weight.
More importantly, this means a reduction in rotational mass, which means you need to expend less energy to rotate a lighter set of wheels.
Of course, the latex tire sealant is designed to plug pin holes from nails, glass, and other roadside debris. Who doesn’t LOVE fewer flat tires!? Larger punctures can usually be "plugged" without removing the wheel from the bike, and in minutes you are back on the road!
Selene Yeager relates her personal experience, “My first foray into DK200 (the 2013 Dirty Kanza 200-mile gravel grinder), I was still running tubes, as was most of the field since tubeless technology hadn’t really taken off yet. Despite running 60 PSI and babying every rocky section and cattle grate, I still pinch flatted a little more than halfway through. The second time, I ran tubeless with a cushy 30 PSI and rolled start to finish, barreling through rock beds and stream crossings, with no tire trouble.”
Today, most quality bikes are equipped with rims that are TUBELESS COMPATIBLE. Most bikes are sold with inner tubes, but the tires and rims are easily converted to tubeless with the addition of sealing rim tape, a tubeless valve, and tire sealant.
Back to our original example but substituting tubeless tires:
A 110 pound woman may now need only 23 PSI while a man with gear totaling 200 pounds may require 36 psi while riding the same rims on the same gravel course. This is still the same 50% air pressure differential for the same tire on the same route, but the comfort level, speed and efficiency are increased exponentially.
Competitive riders agree: Smooth is FAST!
When you consider that gravel cycling involves continual vibrations, bumps, and jolts it isn't hard to understand that a smoother ride will be less tiring, and therefore faster.
In recent years, this has translated into larger tires run at lower pressures. While the early gravel racers used to ride on higher pressure tires in the 30mm range, the most popular width today is 42 to 45mm, with some riders choosing 50mm tires for the roughest courses.
Again, tire width seems to have little impact on speed on gravel courses.
Let's compare two racers who both weigh 150 pounds:
Rider One on 30mm tires might be running at 36 PSI (or more) and bouncing off sharp rocks. Rider Two running 45mm gravel tires might be aired down to 28 PSI (or less) and gliding over the same course, while expending less energy.
The same advantages also apply to loaded bikepacking. A 220 pound combined weight (rider, bike, and gear) might require 50 to 60 PSI on a bike with 32mm tubeless tires. But the same 220 pound weight will require only 30 PSI on 50mm tubeless gravel tires.
Now it is time to discuss tire tread based on the gravel road or trail conditions.
Every event is labeled a “gravel grinder” but there is no standard definition of “gravel”!
Some rides include sections of rough single track. Most have varying combinations of dirt, sand, loose or hard-packed gravel and pavement. The same route can vary tremendously in different seasons and weather. The Mid-South Gravel in Oklahoma is notorious for deteriorating into a mud bath when it rains. The Unbound Gravel is famous for chewing up tires in the Flint Hills of Kansas.
Neil Shirley is a former professional road racer on the National and International circuits. More recently, he has become a gravel cycling convert and is a past champion of the Gravel Worlds in Nebraska. Using his experience, he created the “Industry Standard Guide to Gravel” (ISGG), and it has been adopted by event promoters and cyclists alike.
For a complete description of the various categories, see “Making Gravel Standard” at GranFondoGuide.com. Here is a summary of the categories with the size tires that Neil recommends for each. Ken Avery of Vittoria Tire has added the tread recommendations:
Category 1 Gravel: Smooth, well-maintained dirt/gravel roads.
The crushed gravel secondary roads in the Midwest and "rail trails" are often hard packed and suitable for road bikes with tires as narrow as 25mm (1-inch) – when dry. The original 10-speed bikes had 1.25” tires (32mm) and logged millions of miles on these roads before “mountain bikes” or “gravel bikes” were conceived or named. Modern gravel tires with a slick center tread and light texture on the shoulders up to 35mm are perfect. See the Vittoria Terreno Zero below:
Category 2 Gravel: These lesser-traveled roads have packed tire tracks, but loose dirt and gravel between.
Expect lots of potholes, washboard, and loose dirt on curves – all requiring more tread and more finesse. The centerline needs a light texture, with more pronounced tread on the shoulders. Traditionally, tires of 35mm to 40mm are most common to provide protection against pinch flats and cushion in rough sections. Today, many riders insist on 45mm or wider tires! Avery says, “I use a fish scale design in the center. When you’re rolling, the scales are flat and smooth, but when you hit the brakes, those edges stand up to give you traction. See the Vittoria Terreno Dry below:
Category 3 Gravel: These “Jeep” roads may be mapped but are seldom maintained.
Now we are crossing into MTB terrain where you will encounter deep, eroded ruts and exposed rocks, stretches of sand washed or blown over the trail. If it has been wet or thawing, expect puddles, runoff, creeks, and plenty of mud. Your tires must be wider for traction and protection from the obstacles. Plan on aggressive tread and 40mm to 50mm widths. Also note that many racers run a NARROWER tire on the back wheel in mud! For muddy roads, Avery has these recommendations: “A good mud tire has a mixed tread, with the knobs tighter in the center so it still rolls faster, but with more spaced out toward the outside.” See the Vittoria Terreno Mix below as an example of a tire that can handle all conditions:
Category 4 "Gravel": These routes hardly qualify as roads. We are describing forest trails, ATV trails and mountain bike single tracks.
Your bike must roll over washouts, ruts, tree roots, rockslides, flowing water, snow, and slush – all while climbing or descending sharp hills. These primitive trails are also a key element in most ultra-endurance events. You need wider tires for traction and cushioning. Look for 700c tires from 45mm up to the limits of your frame and fork.
Under these conditions, gravel racers will often share the event with mountain bike riders. Typically, mountain bike tires start at about 55mm (2.2”) and get wider. Still, on long ultra-endurance events, drop bar gravel bikes have advantages in comfort, weight, and aerodynamics. To fit the largest tires, gravel bike frames must fit 27.5 wheels.
The fast-rolling 27.5 x 2.25-inch Maxxis Ardent is a favorite of endurance racers as well as wilderness bikepackers. This tire size is at the narrow end of the tire spectrum for mountain bikes and one of the widest tires that will fit some (not all) gravel bikes.
For hard-core singletrack, most riders opt for a traditional hardtail or full-suspension mountain bike. But for racing on sandy or muddy routes, and self-contained but light bikepacking, a drop bar "gravel bike" with wide 27.5” wheels is lighter and faster.
These wide tire bikes with drop bars have been dubbed “Monster Cross” for blurring the line between mountain bikes and gravel bikes. Some gravel bikes can now be fitted with suspension forks. To reach the same objective, a few MTBs have been converted to drop bars!
There are always exceptioons to these "rules" for gravel bike tires:
Novice riders will be more comfortable moving up to the next larger tire size and more aggressive tread for increased traction and stability.
Use local information. Have the roads recently been graded? Have recent rains exposed sharp ledge rock and roots or added layers of sand and mud? When in doubt, err towards a larger and softer tires.
It is better to use a narrower tire in the mud. You want to slice through the top layer into the firm base. (Remember, cyclocross racers have surviv with narrow 33mm knobby tires.) A fatter tire will squirm and the reduced tire clearance will quickly jam the frame with mud, stopping you completely if not wrecking your drivetrain. The opposite is true in sand, where wider tires offer better floatation and traction.
There is no law that requires you to run the same tread on the front and rear tires. After all, your bike is “one wheel drive” and you may like a stiffer, heavier tread on the rear wheel, paired with a smoother tread for the front tire. Some MTB tire brands have specific front and rear tread patterns.
Wider tires have a greater range of volume, permitting you to “air down” on soft or rocky sections – then add air for hard packed or paved sections. Again, if in doubt, choose the next larger size tire. But DON'T air down inner tubes much or you will invite pinch flats.
Each event or tour may encounter many road surfaces and combinations of conditions. In the end, the best you can do is play the percentages. Determine what are you expecting to see the MOST of – then gear up accordingly.
Don’t make your decision based on the worst short sections of gnarly trail. This means that you may be totally unprepared for some stretches. The goal is simply to be equipped for 98% of what you expect to encounter. On a multiday tour, you can’t change tires unless you find a bike shop. For the unrideable hike-a-bike sections, well, “Embrace the Suck!”.
I recently completed a fast, 120-mile bikepacking loop with over 50-miles of crushed rock roads. While my 700c x 32mm touring tires were perfectly capable in terms of traction and load carrying, I longed for wider 40mm tires for a smooth and supple ride on recently graded sections. The 40mm "gravel tires" would have been equally fine on the paved roads!
Of course, the discussion of gravel cycling tires would not be complete without discussing flats – which can still occur with tubeless tires. Here are some Pro Tips:
Tubeless tire sealant is a thick latex liquid that dries out over time. You need to check the sealant every few months (and before traveling to every gravel grinder), then refresh if necessary. Adding and checking your sealant requires a valve core tool.
For extended tours, carry a small bottle of sealant to add after multiple flats.
Tire sealant won’t clog a big puncture. For big holes, carry a tubeless plug kit. The plug is a soft, sticky rubber"worm" that closes the hole and allows the liquid sealant to fill the remaining gap.
If all else fails, install a tube. Carry one of the tubes you saved when converting your tires to tubeless. It’s also a good idea to carry a patch for a slashed tire sidewall, although a dollar bill works remarkably well! To make roadside repairs, you will also need to carry two or three tire levers.
Don’t forget a tire pump. Some riders swear by CO2 cartridges, but you still need a pump - or a rechargable inflator! Might as well add a few inner tube patches, just in case.
*There is one minor downside to tubeless tires. Some cyclists consider gravel bikes to be the ultimate commuting vehicle. The added frame clearance allows both wider tires and mud guards. For warm climates, tubeless tires are perfect. However, if you are a winter commuter, you will need special cold-weather sealant that won’t freeze. The good news is you can run the “anti-freeze” sealant year round. This sealant is now commonly used in fat bike tires on snow, at temperatures below zero! But you should be checking and refreshing your tubeless sealant at least every six months anyway.
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