Gravel riders need to classify their surface before setting pressure. Going one grade rougher drops your target pressure by approximately 3 psi on 40 mm tyres. The difference between fine packed gravel (Class 2) and chunky loose gravel (Class 4) is 8–10 psi — big enough to transform a sluggish handling bike into a confident one.
The Class 1–5 surface system
Gravel roads exist on a spectrum from nearly-tarmac to rock-garden chaos, and a single "gravel" setting in a pressure calculator can't capture that range. The most useful classification system borrows from US Forest Service road grading, adapted for cycling:
Class 1 is smooth, well-maintained doubletrack or hardpacked fire road. Think canal towpaths, dry farm tracks, or groomed Nordic ski paths in summer. The surface is firm, even, and allows speed. Pressure here should be closer to the high end of your gravel range — maybe 2–3 psi below what you'd run on tarmac.
Class 2 is fine gravel — small particle size, reasonably compacted, with minor loose surface material. Most "gravel roads" on popular routes fall here. Standard calculator "gravel" settings are calibrated for this grade.
Class 3 is mixed, with embedded rock, ruts, and sections of loose material. Belgian paving, rough forest tracks, and typical British bridleways fall here. Drop 3–4 psi from your Class 2 starting point.
Class 4 is loose, large-particle gravel or broken rock. Think alpine descents, unmaintained mining roads, or technical canyon tracks. Another 3–4 psi below Class 3 — this is where traction and control dominate.
Class 5 is anything involving significant loose surface over rough substrate — shale, scree, root networks, hardpack with loose over the top. Pressure here follows MTB logic more than gravel logic.
How to read a surface in 30 seconds
You don't need a geology degree to classify gravel accurately. Three questions cover 90% of cases:
First: can you hear your tyres? On Class 1 and 2, tyres run quietly. On Class 3+, you hear a distinct crunching as particles move underfoot. If you can hear the surface clearly at moderate speed, you're on Class 3 or above.
Second: does the surface hold your footprint? Press your finger into the gravel. Class 1 and 2 surfaces are compacted enough to leave little or no impression. Class 3 leaves a partial print. Class 4 and 5 leave a clear, deep impression — the surface is mobile.
Third: what's the particle size? Hold a handful. Class 1–2 gravel fits comfortably in a closed fist with no sharp edges. Class 3–4 gravel has larger pieces, often angular. Class 5 material is awkward to hold in one hand.
Combine the three answers and you have a reliable classification. For multi-surface routes, set pressure for the dominant surface type, not the smoothest — you'll be riding most of the time on the terrain you're targeting.
Pressure by surface class
For a reference 75 kg rider on 40 mm tubeless gravel tyres, here are the approximate target pressures per class derived from the calculator tools:
Class 1 (smooth doubletrack): 38–42 psi. You're in borderline road territory; compliance matters less than efficiency.
Class 2 (standard fine gravel): 33–38 psi. This is the calculator default for "gravel." Use the consensus number directly.
Class 3 (mixed embedded rock): 29–33 psi. Drop 4–5 psi from your Class 2 number. The tyre needs to conform around obstacles rather than bounce off them.
Class 4 (loose large gravel): 25–30 psi. At this pressure, traction and control are the priority. Rolling resistance doesn't matter if you're sliding sideways.
Class 5 (technical loose/scree): 22–26 psi. Now you're in MTB pressure territory. Wide tyres at very low pressure maximise contact patch and traction.
These ranges assume 40 mm tyre width. For 35 mm, add 4 psi across all classes. For 45 mm, subtract 4 psi. For 50 mm, subtract 8 psi.
Mid-ride pressure adjustments
Most gravel rides involve more than one surface type. A sensible strategy: set pressure for the dominant terrain and know your adjustment range.
Carrying a mini pump for mid-ride adjustments used to be theoretical advice — nobody actually stops to add 4 psi on the fifth kilometre. But with tubeless and the growth of adventure gravel (multi-hour mixed surface rides), this has become practical. A 4-psi adjustment takes 30 seconds with a quality mini pump.
The most useful single adjustment: when transitioning from a long hardpack or tarmac section into technical loose terrain, drop 3 psi at the start of the descent or technical section. You'll feel the difference immediately in traction and steering confidence. If the terrain goes back to smooth, add the pressure back.
One reliable field marker: if your front wheel is deflecting off loose rocks rather than tracking around them, you're 3–5 psi too high for the current terrain. If the bike feels vague and wallowy in turns, you're at or below optimal.