What’s the best tyre size for gravel riding? 40mm and 50mm tyres tested back-to-back

Huge tyre clearances are one of the hot trends for gravel bikes, but the question many riders are asking is, are wider gravel tyres actually faster?

As the gravel category has grown, so have tyre clearances. Gravel bikes cover a broad spectrum but, at the ‘any terrain goes’ end of that spectrum, clearances upwards of 50mm are now common. We’re heading into mountain bike territory here.

So, what’s the best tyre size for gravel riding? Well, the answer’s complicated and depends, as ever, on where and how you ride – but matching an appropriate tyre size to your riding intentions is one of the most important factors in gravel bike setup.

To shed some light on the issue, I tested 40mm and 50mm tyres back-to-back. First, I rode a test lap to challenge the bike over a mix of terrain, so that I can comment on how the bike rides with the two different tyre sizes.

Then I headed to a new location to complete a series of timed roll-down tests to find out which tyre size is the fastest.

Between these two tests, I’ll aim to shed some light on the tyre size conundrum for gravel riding, but first, let’s take a closer look at the test mule.

The bike

To help me carry out this testing, Lauf sent over its Seigla gravel bike.

To test wide tyres, you’ve first got to have space for them in the frame and the Seigla can comfortably accommodate our tested sizes. In fact, the Seigla, launched last year, has space for 57mm (or 2.52in) tyres on 700c rims.

Given that I would have to get myself back up to the top of our roll-down test hill, the lightweight carbon frame would come in handy, as would the 10-44t cassette on the SRAM Red XPLR groupset.

Lauf supplied the bike with two sets of wheels and tyres, enabling me to quickly swap over to what was needed for all 24 runs of the roll-down test.

The wheels themselves were e*thirteen’s XCX Race carbon gravel wheels. With an internal rim width of 24mm, they’re an ideal match for the latest gravel bike tyres.

In terms of the rubber, I opted for a set of Maxxis Rambler tyres, chosen because they're a relatively common choice for all-round gravel riding. They have a tight tread pattern, well-suited to the roll-down test, while the 120 TPI, tubeless-ready bead makes for a supple combination capable of handling the low testing pressures. Crucially, they’re available in 40mm and 50mm sizes.

The test protocol

The lap test took in a variety of off-road terrain, including loose corners, a steep pitch and muddy ruts. Here, just one variable – tyre width (40mm and 50mm) – changed between the two laps.

For this test, I optimised the tyre pressure for each width using Silca’s pro tyre pressure calculator, which takes into account system weight, gravel surface and a range of other influencing factors.

For the roll-down test, two variables were tested: tyre width (40mm and 50mm) and tyre pressure (25psi and 35psi). I also tested each combination on two lines for the roll-down test: a smooth line and a rougher line.

The aim here was to establish the fastest combination for gravel riding. It’s worth noting, however, that, while rolling resistance is often the main focus, the weight, aerodynamic profile and casing tension of the two tyres would have an impact on performance, too.

I rode each line three times – making for 24 runs in total – to provide an average time for each setup.

Now, this is a real-world test, not a lab test, but for both the lap test and roll-down test, as many variables as possible were kept the same. This is key to being able to spot differences in whatever it is you’re testing, enabling you to be sure your results haven’t been influenced by external factors.

Given the nature of testing in the real world, especially during spring in the UK, finding a weather window to allow for consistent conditions was tricky.

The pictures you see of the roll-down testing were shot on our designated filming day. The actual testing took place on a far calmer and sunnier day, as you can see from the GoPro footage in our video. It was important to record the data on the calmest day possible to keep it as consistent as we could.

Lap test

I kicked things off by riding a back-to-back lap test, on a trail designed to push the tyres, and myself, fairly hard so I could get a feel for their limitations.

For the 40mm tyres, the rear tyre was set to 42psi, while the front was set to 40psi. For the 50mm tyres, my rear was set to 28.5psi, while the front was run at 27.5psi. The sealant level was kept the same.

I focused this part of the test on ride impressions, and how the tyres felt over a variety of terrain, rather than lap times.

The course was designed to be around five minutes long, with as many surface changes as possible, including the natural features, such as roots and rocks, that many riders will find on an off-road ‘gravel’ ride.

Conditions were perfectly calm and the course was a mix of dry and wet. These conditions remained constant throughout the lap test.

Lap test ride impressions

The first thing I noticed was that the narrower 40mm tyre felt (and I should emphasise ‘felt’) faster on most sections of the course. Off the start line, the bike felt far more sprightly and, on the straight drags that made up the first half of the course, speed felt easier to maintain.

Flying into the wooded sections, I didn’t feel as though the narrower tyre lost much to the 50mm tyre in terms of cornering composure or grip, and on muddy sections I actually found the 40mm tyre to be a bit better.

It seemed to bite down into the mud a little more, finding better traction, whereas the wider tyre floated a little, making it harder to track through ruts.

Through the corners, however, I was far more confident on the wider tyre. It seemed to run wide a few times, but this may be because I felt I was able to leave the brakes alone. Even if I hit debris on the exit of a corner, the wider tyre remained planted.

In contrast, the narrower tyre suffered from a little loss of traction as I winched my way up the steep kicker. Generally, it felt easier to climb off-road technical stuff on the wider tyre because it felt noticeably grippier.

Roots, rocks and any impacts were also a lot smoother to ride on the wider tyres – something that didn’t surprise me at all.

Roll-down test

But what about pure straight-line speed?

Rolling resistance, aerodynamics and weight are the three main forces acting against you during a gravel ride, and the traditional thinking is that wider tyres are slower, because they’re heavier and less aero.

Our test segment was 770 metres long, losing 50 metres of elevation in that time. The left-hand side was heavily rutted, with large stones and some significant impacts. This acted as my rough line.

The right-hand side was our smooth line. For the most part, this was finely graded gravel with only a few patches of small, loose stones.

I started each run rolling at 20kph. At the start line, I began a lap on my Garmin Edge 1040. I didn’t pedal after the start line, kept as close as I could to the same body position for each run, and also tried to keep my line choice as consistent as possible.

Conditions on the day of testing were as good as could be expected for the time of year. I was treated to full sunshine, light winds and beautifully dusty gravel.

Roll-down test results

Smooth gravel

Rough gravel

Having ridden down the same hill 24 times, the first conclusion I can draw is that, in our tests, the data provides a tightly-packed picture, though the fastest average times, for both the smooth and rough lines, come from the 40mm tyre at 25psi.

However, if comparing the 40mm and 50mm tyres at 25psi, there’s only a difference of one-second across both lines. While I controlled as many variables as possible, the results are so close that, ultimately, it’s difficult to conclude, with statistical significance, that one tyre is faster than the other.

Naturally, with real-world testing, getting perfectly precise data is very challenging, so my results should be taken as ballpark rather than gospel.

Still, it’s worth noting that across all eight tyre width and pressure combinations, the lower tyre pressure (25psi) tested faster, on average, than the higher tyre pressure (35psi), regardless of tyre width and line choice.

Interestingly, while a lower pressure tested faster for both the 40mm and 50mm tyres, the lower pressure had, based on our data, a more significant impact on overall speed when using the narrower tyre, with a three-second difference on the smooth line and a five-second difference on the rough line.

It’s also possible that lowering the tyre pressure for the 50mm tyres even further may have netted more gains, too.

This goes a small way to showing how important tyre pressure can be when gravel riding and, based on our test, lower pressures can reduce rolling resistance, even with what’s considered a fairly narrow gravel tyre by today’s standards.

As an aside, across the entire test, you can begin to see the importance of line choice. Gravel riding can, by its very nature, throw up unpredictable terrain, from perfectly smooth roads to root-laden MTB trails, and our testing shows a consistent increase in time when riding the rough line, compared to the smooth line (both of which measured 700m in length on the same section of gravel).

In fact, line choice proved to have the biggest influence on time across the test, with a nine-second (average) time difference between the smooth line and rough line, when using the 40mm tyre at 35psi. Here we can again see the potential gain in running a lower pressure on rough terrain, even on 40mm tyres, though, as I’ll come on to, that’s not without its risks.

There you have it, that’s the data. While the time gaps were small in places, my test run wasn’t overly long, and some clear themes emerged from the testing – both from the data and my ride impressions, so let’s wrap things up.

Which tyre width is right for you?

While the data above is very neat, there is still plenty to be said about what the numbers can’t tell you.

Firstly, the 40mm tyre on the rough part of the roll-down test was noticeably harsh over the course of the day. My hands were shaken to the point that I couldn’t actually feel them and the bike started to feel incredibly skittish when travelling at high speeds.

The higher pressures on the rough line were also fatiguing, and this is reflected in the average times in the data. Beyond the data, at the end of each run I had to ride back up the hill and I felt very sluggish for a minute or so after each high-pressure run on both tyre sizes.

If you were looking at a race on rough or varied terrain, or even just a ride situation that included some descents into climb – a common feature of a rolling course – experimenting with a lower pressure could help to reduce fatigue throughout the day.

Generally, lower pressures contributed to a far calmer experience, and produced faster times, and wider tyres also went some way to improving my on-bike comfort.

That said, those lower pressures don’t come without their pitfalls, and there’s always a balance to be struck. Narrow tyres at low pressure can be more prone to punctures and it felt as though I was going to damage a rim on one of my three runs, when using the 40mm tyres at 25psi on the rough line.

My test bike was set up tubeless but, if you’re using inner tubes, the puncture risk is further increased.

So what’s my conclusion from all of this?

For terrain that’s at the smoother end of gravel, narrower tyres (around 40mm) would be my choice. My testing suggests an increase in speed here, though the difference was very small.

For chunkier gravel, or heavily rooted forest sections, however, the comfort, composure and puncture protection of a wider tyre (be that 42mm, 45mm, 50mm as tested or beyond), run at a lower pressure, really comes into play. For the majority of my off-road riding, that’s where I’d settle.

For general riding, particularly if riding predominantly off-road here in the UK, where ‘gravel’ can take in all types of testing terrain, the jump in ride quality and control is significant, without any significant impact on straight-line, off-road speed.

It’s also worth emphasising, once again, the importance of tyre pressure, alongside tyre width.

There are lots of variables to consider when ‘setting’ a tyre pressure – including but not limited to, rider and bike weight, tyre choice, terrain, weather/trail conditions and whether you’re running inner tubes or not. Taking some time to experiment with a lower pressure than usual could pay off.

Oh, and ride a smooth line.