The Quest for the Perfect Off-Road Tire

What Rolling Resistance Testing Actually Tells Us About Tires

close up of mountain bike tire tread on dirt.

Most tire marketing is designed to make you feel like you're leaving speed on the table. Run this tread compound, this casing, this width, and you'll be faster. The numbers look compelling. The white papers sound rigorous. And then you go ride the tire in actual conditions and wonder why nothing matches what you were promised.

I recently sat down with John Karrasch on Training Babble to dig into this. John is a bike fitter and coach based out of Birmingham, Alabama, working out of Cobb Cycles. He's also spent the last three years doing something almost nobody else is doing: real-world rolling resistance testing on gravel and mountain bike tires, conducted outside on actual terrain rather than on a steel drum in a lab.

The conversation covered more ground than I expected. Here's what I took out of it.

The problem with existing tire data

Most of the rolling resistance data available to athletes comes from one of two places: manufacturer white papers or sites like bicyclerollingresistance.com. The manufacturer data is optimized to show where their product excels, tested at specific yaw angles and speeds that may not reflect real-world riding at all. The independent drum testing is more honest but uses a smooth steel cylinder that has nothing in common with gravel or dirt trail surface.

What John does instead is ride loops on actual surfaces, using power and speed data analyzed through the Cheung method, also called virtual elevation, to derive a coefficient of rolling resistance (CRR) value that can then be translated into watts. He tests on multiple surfaces, from smooth pavement to rough gravel to chunky mountain bike trail, so the results build progressively and reflect how tires actually perform across a range of conditions.

The tests themselves are shorter than you'd expect, around ten minutes per tire per surface, but because he's riding repeated loops he can verify consistency in real time. If one lap looks weird because of a wind gust or a line choice mistake, he sees it immediately and accounts for it.

The results don't always match what the drum tests show. That's the point.

Wider is faster. This needs to stop being a debate.

One of the clearest findings from John's testing is something that experienced off-road riders have felt for years but struggled to prove: on rough terrain, wider tires win. Not by a little. By a lot.

His rougher gravel and mountain bike trail results show that beyond a certain threshold of surface roughness, tire width matters more than almost anything else. A 2.1 or 2.2 tire from a premium brand with excellent rolling resistance numbers on smooth pavement gets beaten on rough trail by a 2.4 with more volume, almost regardless of other variables.

The reason is simple. A wider tire can conform to the terrain rather than bouncing over it. When a tire is too narrow or too firm for the surface, energy that should be going forward goes upward instead. The rider feels this as vibration and choppiness. They often interpret it as speed because the feedback is intense. It usually isn't speed. It's energy loss.

The old belief that narrower is faster comes from road cycling, where smooth pavement rewards a narrow, hard contact patch. That logic does not transfer to dirt. A rider running a 2.4 at appropriate pressure on chunky trail is almost always faster than the same rider on a 2.2, even if the 2.2 has better rolling resistance numbers on a drum test.

Tire pressure: there is no magic number

The pressure question is one John hears constantly. What PSI should I run?

His answer, and mine when athletes ask me the same thing, is that it depends on your weight, your bike, the surface, the conditions, and the temperature. There is no universal answer. What John's testing has shown is that there's a window of about plus or minus two PSI around the optimal pressure where rolling resistance stays roughly the same. The tire feels different at the edges of that window, but the speed impact is minimal.

What that means practically is that you have more room to optimize for handling and comfort than people realize. You don't have to choose between a fast-rolling tire and a tire that corners predictably. In most conditions, you can have both, as long as you're somewhere in that window.

The complicating factor is that the window moves throughout a ride. Temperatures rise, altitude changes, and tires heat up with use. A pressure that felt right at 6am in Emporia can feel completely wrong at noon on a rocky climb. John's advice is to get good at reading your tires by feel, learn what they feel like when they're slightly low or slightly high, and be willing to adjust mid-ride. The most sophisticated gauge in the world doesn't replace the knowledge of what your tires feel like at the right pressure.

I've started thinking about this the same way I think about RPE. The number is a guide. The feel is the signal.

The fastest tire is the one that keeps you on your bike

This is something I say to athletes regularly and John confirmed it from a testing standpoint. Rolling resistance is not everything. A tire that tests fast on pavement but flats every other ride is not a fast tire. A tire you don't know how to handle in sketchy conditions is not a fast tire.

John made a point that stuck with me: changing tires constantly limits your ability to learn what you have. Every time you switch, you're starting over. You don't know how the new tire will handle in wet rocks or loose over hard or deep sand. You don't know what it feels like when it's slightly under-inflated or what the early signs of a flat feel like with that casing. That familiarity is worth more than a few watts of rolling resistance advantage.

I tell athletes the same thing about fitness. Consistency beats optimization. Pick a tire that works reliably for your terrain, learn it thoroughly, and stop looking at what everyone else is riding.

On the subject of tire changes before big events: John's experience and my experience align completely here. The last-minute switch to a supposedly faster tire has ended more races than it has won. Run what you know. Save the experimentation for training.

Compound and casing matter, but not in the way marketing suggests

John has tested enough tires now to see where compound makes a real difference and where it doesn't. The Continental Race King in Black Chili versus Pure Grip is one example he mentioned: the Pure Grip compound is dramatically slower in his testing, and you can hear it in the way it kicks rocks off the trail. For most riders on most terrain, a fast-rolling compound with moderate grip is the right choice.

Casing is the variable he thinks about most when helping riders pick tires for their specific conditions. A supple casing conforms better and can run lower pressure, but it requires more care about where that pressure sits. Run it too low and it folds in corners. A more rigid casing with a puncture protection layer needs higher pressure to handle well but is more forgiving when that pressure drifts.

The broader lesson from everything John tests is that the best tires are the ones where everything works together. The casing, compound, tread pattern, and volume feel cohesive rather than like a collection of individually optimized components. He cites the Continental GP5000 on the road side as an example of a tire that dominates not because any one element is exceptional, but because the whole thing is designed well. The best off-road tires have the same quality. You can't point to one thing that makes them good. Everything just works.

How to pick a tire for your season

If you're racing regularly in one area, John's advice is practical and worth repeating. Pay attention to what experienced local riders are running. Talk to people at races about what's working and what's flatting. Talk to the staff at your local shop, because they hear everyone's stories and accumulate a lot of concentrated real-world knowledge.

Don't copy what professionals run unless the conditions at your local races match where those professionals raced. A World Cup XC tire designed for hardpack European singletrack is not automatically the right choice for Colorado front range trail. The terrain matters.

And if you want to do your own quick and dirty comparison test, John's suggestion is to start on pavement. Even for off-road tires, smooth pavement testing with a power meter and a speed sensor riding consistent loops gives you a useful baseline. It won't tell you everything, but it'll tell you something, and it's a lot easier to control the variables than testing on trail.

John shares his tire testing results and thoughts on Instagram at flexfitbyjohn and has more detailed analysis available through his Patreon and on his website at johnkarrasch.com. If you want to go deep on this, those are worth following. He's doing work that nobody else is doing in this space and the results are genuinely useful for anyone trying to make smarter equipment decisions.

The full conversation is on Training Babble.