How much slower are gravel tyres than road tyres?

We look at the differences between a road tyre and a gravel tyre in terms of efficiency and speed

Clock11:36, Friday 24th November 2023
How does a gravel tyre (left) stack up against a road tyre?


How does a gravel tyre (left) stack up against a road tyre?

If you've ever tried to keep up with road cyclists on a gravel bike, you'll know that gravel tyres roll slower than road tyres. Gravel tyres have been developed to offer the best balance in performance between speed, control and comfort on loose surfaces. To do this, they are wider, more knobbly, and usually made of a softer rubber compound. On the tarmac, though, it makes them feel slow.

The question is, exactly how much slower are gravel tyres than road tyres on tarmac? To find out, we put a road tyre and gravel tyre head to head.

Representing the road, we used the 28mm Continental GP500, and for the gravel, we used a set of 40mm Continental Terra Speed, a typical, medium-level treaded gravel tyre. Here's what we found.

What makes a tyre fast or slow?

The main factor that contributes to how fast a tyre rolls is its rolling resistance. This is the force that it takes for the tyre to move over a road surface. As you ride along, the tyres of your bike deform slightly to meet the surface of the road. That process of deformation takes energy, which is why, traditionally, we've always thought that narrower tyres, pumped up to higher pressures, gave lower rolling resistance.

Out in the real world, however, it is not that straightforward. Alongside the energy lost to that rubber deformation, energy is lost to vibration, as your bike bumps up and down over imperfections in the road. Surprisingly, even on smooth tarmac, the energy losses caused by vibration can quickly exceed those of tyre deformation, especially on narrow, high pressure tyres.

That's why, in recent years, everyone, including road racers in the professional peloton, has started using wider tyres with lower pressures.

But there's a limit to this 'wider is better' mentality. At a certain point, a tyre will become so wide that the energy losses due to that process of deformation begin to outweigh the advantages that come with reduced vibration losses. On the road at least, the optimal middle ground between low rubber deformation losses and low vibration-dampening is typically accepted as around 28mm. In fact, 28mm has been proven to be the fasted tyre width.

A wide gravel tyre, then, will be way off this ideal middle ground, so the losses from rubber deformation will contribute to that 'draggy' feel, out on the road.

Then there's the matter of tread pattern. Most road tyres will have a slick or semi-slick profile. This means that the surface of the tyre is smooth or might have some light patterning on the sides. These add grip in the wet and loose surfaces without affecting rolling resistance.

A typical gravel tyre, meanwhile, will have a more pronounced tread. Sometimes, this will be a file tread, and sometimes it'll be almost like a mini mountain bike tyre, with aggressive knobs for gripping on mud and rocks. On the road, that tread is going to increase the rolling resistance further, as it means the tyre has to 'bump' over each piece of tread as it rolls.

Indoor testing

To get a gauge of the difference between the two tyres, GCN's Ollie Bridgewood conducted a test.

First of all, the two tyres went head to head on a set of rollers, using a tyre pressure of 70psi for both the road and gravel tyres. This shows the difference between the tyres' rolling resistance in isolation.

Then, we tested the tyres at the kind of pressure people would actually use them at. So, 90psi for the road tyre and 40psi for the gravel tyre.

To test rolling resistance, we rode on both tyres at 45kph, and observed the different power that the rider had to put in to maintain it. The more power it takes to achieve the target, the slower the tyre.

The road tyre

At the control pressure of 70psi, it required 327W to hold the target speed of 45km/h. When the pressure was increased to a more representative 90psi, the power needed dropped significantly to 299 watts

The gravel tyre

At the control pressure of 70psi, the 40mm tyre required an output of 449W to achieve the 45kph target. Dropping the pressure down to a more realistic 40psi, the resistance increased significantly with a monumental output of 516 watts necessary to hold the target pace.

Comparing the results

When comparing the results from the indoor testing, the difference between the tyres is staggering. At the 70psi control pressure, the difference between both tyres was 122W, meaning it took 28% more power to maintain that speed on the gravel tyres.

However, when we used the realistic pressures - 40psi for gravel and 90psi for road - the effort required differs by 217 watts (42%).

With all variables removed, it appears that a gravel tyre is much slower than a road tyre, but it isn’t that simple.

In the real world, the effects of vibration are significant and can easily cancel out some of the advantage the road bike has on a set of rollers.

What is the difference out on the road?

To find out what the difference is out on the road, we conducted a simple roll test. To do so, we saw how far we could roll on each tyre, from a standing start, using just a gentle slope to gain momentum.

The question was: how far would the tyres roll once the slope ended? From that, we could get a sense of just how differently these tyres roll out on the road.

To remove as much error as we could, we performed both tests a few times and found an average.

The results from this testing showed that, on average, the road tyre rolled 19.8 metres further. In the testing parameters, this is a sizeable difference and shows that even on real tarmac, a road tyre is a lot more efficient.

What is the difference on light gravel?

The results so far might not come as a shock, however when you move onto some gravel surfaces, you would expect the gravel tyres to come into their own.

To find out, we performed another roll test, this time on a light gravel track. Which one would roll the furthest?

Surprisingly the road tyre still outperformed the gravel tyre here – the gravel was still pretty smooth, meaning the vibration losses weren't so great as to put the gravel tyre in the lead. The margin between them was significantly reduced, though. This result is important for road riders who might stray onto some light gravel every so often. Even when the road runs out, a road tyre can still be the fastest choice.

Other considerations

These tests only took into consideration the rolling resistance of the tyre on fairly groomed surfaces. On rougher surfaces, the vibration dampening of gravel tyres would allow them to roll faster than road tyres.

Additionally, gravel tyres offer more grip for both cornering and braking, and better puncture protection. So when off-roading, they'd allow you to ride much faster.

So how much slower are gravel tyres than road tyres?

Gravel tyres have a measurable penalty over a road tyre in terms of their outright efficiency. There are other factors at play however. The gravel tyre is designed for very different needs from a road tyre, and on the right terrain, they excel.

If you're trying to decide which tyres to fit to your bike, you need to think about what kind of riding you do, and what your goals are. Road bike tyres are faster on tarmac, but for rough surfaces, gravel tyres are faster, more controlled, and more comfortable. For winter riding, gravel tyres can offer more grip and better puncture protection.

If you are looking to ride in a chain gang or take part in a local time trial gravel tyres are going to slow you down, but it is down to you to decide if that's important.

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