The Gravel Tire Pressure Testing That Changes Everything
New wind tunnel and rolling resistance data reveals how rim width and tire pressure interact to affect gravel bike speed and comfort.
Silca just released the most comprehensive gravel tire pressure testing ever conducted, and the results will make you rethink everything about how you set up your bike. The data, collected across 47 tire and rim combinations and published May 27, shows that optimal pressure varies by up to 12 psi depending on rim internal width—and running the wrong pressure can cost you 15-20 watts on mixed terrain.
The testing combined rolling resistance measurements on Silca's laboratory drums, wind tunnel analysis at the A2 tunnel in North Carolina, and real-world vibration testing on gravel roads near Silca's Indianapolis headquarters. The headline finding: wider rims allow lower tire pressures without increasing rolling resistance, but only if you account for tire casing construction and tread pattern.
The Rim Width Variable
Silca tested three rim internal widths—21mm, 25mm, and 30mm—with five popular gravel tires ranging from 38mm to 45mm measured width. On a 21mm internal rim, a 42mm tire reached optimal rolling resistance at 38 psi (for a 75kg rider on smooth pavement). But mount that same tire on a 30mm rim, and optimal pressure dropped to 32 psi for equivalent rolling resistance.
The mechanism relates to tire shape and contact patch geometry. Wider rims support the tire casing more effectively, creating a rounder profile that reduces sidewall deformation during rotation. Narrower rims create a lightbulb-shaped profile where the tire sidewalls collapse inward, increasing hysteresis losses as the casing flexes through each wheel revolution.
On rough gravel surfaces, the rim width effect became even more pronounced. The 42mm tire on a 30mm rim at 32 psi was 7.4 watts faster than the same tire on a 21mm rim at 38 psi over a 100-meter section of chunky gravel with 8-12mm aggregate. The wider rim and lower pressure combination allowed the tire to absorb impacts rather than bouncing over them, reducing energy losses from suspension motion.
The Aero Penalty
Lower pressures improve rolling resistance and comfort, but they come with an aerodynamic cost. Tires at lower pressures bulge more at the contact patch, increasing frontal area and disrupting airflow around the rim. Silca's wind tunnel data showed that dropping from 38 psi to 28 psi on a 42mm tire increased drag by 3.2 watts at 40 km/h.
The aero penalty scales with speed—at 30 km/h, the drag increase was just 1.1 watts, but at 50 km/h it jumped to 6.8 watts. For most gravel riding where speeds average 25-35 km/h, the rolling resistance savings from lower pressure outweigh the aero penalty. But on fast gravel races with long paved sections or smooth dirt roads, higher pressures may be faster overall.
Rim design matters, too. Rims with external widths closely matched to tire width (a rim-to-tire width ratio of 0.88-0.95) showed 15% less aero penalty from lower pressures compared to narrower rims where the tire bulged beyond the rim profile. If you're running a 42mm tire, a rim with 24-26mm external width at the brake track creates the smoothest airflow transition.
Comfort vs. Speed: The Sweet Spot
Silca used a vibration-measuring accelerometer mounted to the bike's head tube to quantify comfort across the pressure range. Vibration amplitude increased exponentially as pressure rose above the optimal range, with high-frequency vibrations (80-120 Hz) showing the sharpest spikes. These frequencies correlate with rider fatigue and muscle damage during long rides.
The testing identified a comfort-speed sweet spot: pressures 4-6 psi below the rolling-resistance-optimal value delivered 97-99% of the speed while reducing vibration amplitude by 22-28%. For a 75kg rider on 42mm tires on 25mm internal rims, that meant running 33-35 psi instead of 37 psi. You sacrifice 2-3 watts of rolling resistance, but you'll finish a four-hour gravel race with significantly less muscle damage and fatigue.
The Tire Construction Factor
Not all tires respond to pressure changes identically. Silca tested tires with three casing types: supple 120 TPI casings, standard 60 TPI, and reinforced casings with additional puncture protection layers. Supple casings reached optimal rolling resistance at 3-5 psi lower pressure than standard casings, and they maintained that advantage across the entire pressure range.
A 42mm supple-casing tire on a 25mm rim showed 8.2 watts lower rolling resistance at 35 psi compared to a standard-casing tire at the same pressure. The supple casing deforms more easily, reducing hysteresis losses. But supple casings are more vulnerable to punctures and sidewall cuts, so the choice depends on terrain and risk tolerance.
Reinforced tires required 4-6 psi more pressure to achieve equivalent rolling resistance, but they still outperformed standard casings in comfort metrics at the same pressure. The additional material damped high-frequency vibrations, even though it increased overall stiffness.
Pressure Guidelines by Rider Weight
Silca developed a pressure calculator based on the test data, adjusted for rider weight, rim width, tire size, and casing type. For a 70kg rider on 42mm supple-casing tires with 25mm internal rims, the recommendations are:
- Smooth gravel/hardpack: 33-35 psi front, 35-37 psi rear
- Mixed gravel/moderate chatter: 30-32 psi front, 32-34 psi rear
- Rough/chunky gravel: 28-30 psi front, 30-32 psi rear
Front tire pressure runs 2-3 psi lower because less weight sits on the front wheel, and lower front pressure improves steering traction and compliance. For every 10kg of rider weight, add or subtract 2 psi from these baselines.
What This Means for Your Riding
If you're running narrower rims (21-23mm internal), you're leaving speed and comfort on the table unless you're running higher pressures than optimal for wider rims. Consider upgrading to 25-28mm internal rims to unlock the benefits of lower pressure without rolling resistance penalties. The investment—$600-1,200 for a quality wheelset—pays off in reduced fatigue on long rides.
Experiment with pressure in 2 psi increments rather than making big jumps. The Silca data shows that optimal pressure sits in a narrow range, and going 4-5 psi too low can cost as much speed as going 8-10 psi too high. Start at the calculator recommendations, then adjust based on feel: if you're burping air or feeling rim strikes on sharp impacts, add 2 psi. If you're bouncing off washboard sections and feeling excessive vibration, drop 2 psi.
Don't assume the same pressure works across all tire models. Casing construction varies significantly between brands, so a pressure that's perfect for one 42mm tire may be 5 psi off for another. Re-evaluate pressure whenever you switch tires, and track what works for you across different conditions.