No topic in road cycling products and technology is as full of myths, misconceptions, and rules of thumb as the width and pressure of tires. Over the past decade, road-tire technology has seen enormous changes—much of that stemming from the introduction of road tubeless. This technology has pushed rims and tires wider while enabling riders to run lower tire pressures more safely.
In just the last few seasons, wider tubeless tires have been rapidly adopted by many professional riders and top road teams. The once omnipresent 23mm tire is now virtually nonexistent among pros, having been replaced by 25, 26, and occasionally even 28 or 30mm-wide rubber. For many riders, though, what’s always worked before still works fine. This begs the question: Are wider tires and lower pressure actually faster?
Rather than measuring this in the lab, we set up our test to function in real-world riding conditions like those you’d expect to encounter on a typical road ride. We used the same Pirelli P Zero Race TLR tire in three different widths—26, 28, and 30mm—along with a reliable digital tire-pressure inflator, and a day spent riding, testing, and swapping lots of tires.
Our test featured two timed segments (three runs each, averaged). The first test was on a smooth and flat 2.5-mile section on a bike path. The second test was on a 2.5-mile rolling road with mostly good-quality pavement. For tire pressure, we tested three common combinations: 90 front/95 rear psi (90F/95R psi), 70F/75R psi, and 50F/55R psi. Aiming to be as precise as possible, the only variables we changed were tire width and pressure. The bike, wheels, gearing, clothing, hand position on the bike, and the start/stop of each run were all unaltered. The same 145-pound rider performed all test rides. We used a power meter to pace each effort at 210 watts, which was roughly the effort of riding 20 mph for our tester on a flat road. The pacing of each run was kept close enough to fall under the +/– 1.5% of our power meter’s margin of error, or roughly 3 watts over or under. Each tire and pressure combination was run three times and then averaged.
In our flat terrain test over smooth pavement the fastest set up was a 30mm tire pumped up to 90 front and 95 rear. Second fastest by just two seconds was the same tire at 70 front and 75 rear.
On rolling terrain with mixed pavement quality the fastest set up was the a 28mm tire pumped up to 90 front and 95 rear. Second fastest by just one second was the same tire at 70 front and 75 rear.
Making Sense of the Results With Josh Portner
Real-world testing can produce some messy data, but it beats making assumptions. Plus, we had Josh Portner at our disposal as an expert resource. Josh is a true tire nerd and the former technical director at Zipp. He currently owns the premium accessory brand Silca and runs a company called Aeromind, which offers various consulting and testing services in the areas of aerodynamics, tires, and tire pressure for pro cycling teams. We shared our testing and data with Josh, and he spent some time walking us through making sense of it. The first thing that surprised us was that in both tests, the fastest setups used what we considered to be relatively high pressure, 90F/95R.
Josh said, “On some level, that doesn’t surprise me. We’ve spent 10 years convincing people to lower their tire pressure. Now with a company like Zipp only allowing a max pressure of 72.5 [psi] in their wheels, it feels like the pendulum has swung a bit too far in that direction. We have people come to us and say things like, ‘Your tire pressure calculator recommended 90,’ and they think that’s too high. But the reality is that it’s all about the surface you’re riding on.”
The long-held belief that the higher your tire pressure, the faster you go is true only to a point. Josh refers to that point as the “breaking point.” Up to that breaking point, your rolling resistance will decrease if you increase tire pressure, and that relationship is linear. But once your tire pressure reaches the breaking point for a given scenario, the relationship becomes exponential in the other direction.
Essentially: The more you pump up the tire past the breaking point, the bigger the rolling resistance penalty gets. With that in mind, it’s likely that the surface quality of the roads we were testing on was quite good, and our high pressure of 90F/95R was not higher than the “breaking point” in that scenario. Meaning, that we would have needed to test at higher pressures to find the point at which rolling resistance started to go up.
Okay, but what about tire width?
“There is the conventional wisdom that wider tires have lower rolling resistance, and if you took a given tire construction and just scaled it, you could probably find that to be true. But in the real world, because of the ways tire makers make their tires, this is not always true. I know it can be hard to hear, and I get a lot of grief about this answer,” Josh said with a grin, “but it really just depends.”
So it’s not that surprising that a 30mm tire pumped up to 90F/95R was the fastest on a stretch of smooth, flat road. Putting in 210 watts gave our test rider an average speed of just a hair over 20 mph. At that speed, rolling resistance is not getting overwhelmed by the aerodynamic penalty of the larger tire. Josh put it in the context of the pros racing at the Tour of Flanders: “The fastest tire at Flanders is still the 28, and it’s probably pumped up to more than most people think. That’s because even though the cobbles are pretty rough at Flanders, the riders are going so fast.”
Hidden among all the talk of efficiency, speed, and performance is comfort, which is the X-factor for many riders and the decisions they make about their tire width and pressure. Here our testing proved to be quite reassuring. The fastest setups in both tests used 90F/95R; the second-fastest setups were just one and two seconds slower but at 20 psi less front and rear. While we didn’t have a numerical way to measure that increase in comfort, our tester certainly felt the difference on the road. “So much of that is because of how our bodies are attuned to perceiving things,” Josh chimed in. “We’re just not good at perceiving speed very accurately, but we’re really good at feeling vibrations, and we’ll often use vibrations as a proxy for speed. So that 120 psi that rattles your teeth feels really fast, but that’s just because of how we’re tuned to perceive those sensations.”
The biggest variable that affects your ideal tire pressure (in terms of speed) will be the surface on which you ride. In our case, going down quite a bit in pressure afforded our tester a significant boost in comfort with only a minimal performance penalty.
So based on this testing and talking to Josh, here’s our main recommendation for road riding: If you want to go fast, pumping your tires up is good (to a point), but the efficiency penalty for going with lower pressures to boost comfort is very small. Additionally, the difference between using a 28 vs. 30mm-width tire likely comes down to how the manufacturer makes the tire, more than anything else. Using a 26mm tire only really makes sense if you’re consistently riding at professional race speeds, due to its better aerodynamics.
For the smooth and flat bike path test run, the 30mm tire was the fastest. It recorded the best overall time of 7 minutes, 30 seconds at 90F/95R psi, and the second-fastest time of 7:32 at 70F/75R psi. The slowest time in this test, a 7:47, was recorded using the 28mm tire at 90F/95R psi.
On the return trip test, which was rolling, with a bit more pavement variability, the fastest setup turned out to be a 28mm tire at 90F/95R, which clocked in at 7:16. Again, the second-fastest setup proved to be very close behind, at 7:17, and was the 28mm tire but inflated at 70F/75R psi. The slowest setup in this test was a 30mm tire at 50F/55R psi.