For a 75 kg rider on 40 mm tubeless gravel tyres, the three major calculators range from 30–40 psi. That 10 psi spread reflects genuine disagreement about whether you're optimising for rolling resistance, control, or confidence. This guide breaks down the surface classification, explains the width tradeoffs, and gives you a system for picking your starting number.
Surface classification — the variable the calculators can't know
The single most important input for gravel pressure that no calculator can read automatically is actual surface condition. All three tools accept "gravel" as a category, but gravel encompasses an enormous range — from hardpacked doubletrack that rolls like rough tarmac to loose, rocky terrain that demands MTB-level compliance.
Use a five-class classification: Class 1 (smooth hardpack, low particle size, fast), Class 2 (standard fine gravel, the calculator default), Class 3 (mixed surface, embedded rock, some loose material), Class 4 (large particle loose gravel, chunky tech), Class 5 (technical loose or scree, MTB logic). For each class step up, subtract 3–4 psi from your Class 2 baseline.
Most paved-to-gravel races and popular gravel routes average Class 2–3. The "fine gravel" setting in the calculator targets Class 2; the "coarse gravel" setting targets Class 3–4. If your route is primarily Class 2, use the fine gravel setting. If it involves significant Class 4, use coarse gravel and expect to run near the lower end of the range.
The weather multiplier: wet gravel behaves one class rougher. A dry Class 2 route becomes a wet Class 3. Add 2–3 psi of headroom to your planned reduction when rain is forecast.
Tyre width: where the numbers get interesting
Going from 35 mm to 40 mm gravel tyres drops your optimal pressure by 4–6 psi at the same rider weight. Going from 40 mm to 45 mm drops it another 4–5 psi. These are large steps — the width effect is the most powerful single input in the pressure equation for gravel.
The key reason: pressure exists to maintain a specific contact patch shape under load. A wider tyre achieves the same contact patch at lower pressure. Running a 45 mm tyre at 40 mm tyre pressures gives you an over-inflated, harsh ride and reduces traction. The tyre casing becomes boardlike and bounces over surface irregularities rather than absorbing them.
Volume isn't just about width. Tyre casing construction matters: a supple high-thread-count casing deforms more readily at a given pressure than a stiff protective casing. If you're on a training-grade 40 mm tyre with thick sidewalls, your optimal pressure is 2–4 psi higher than a racing-grade 40 mm supple casing at the same width and rider weight. The calculators can't model casing stiffness, but the feel is obvious on the bike — a stiff casing at too-low pressure feels simultaneously harsh and vague.
How the three calculators diverge on gravel
Gravel is where the calculators disagree most. On smooth road, SRAM and Silca are within 2 psi. On coarse gravel with a 40 mm tyre, the gap can be 6–8 psi. Here's the breakdown for a reference 75 kg rider on 40 mm tubeless gravel tyres:
Silca's surface impedance model produces approximately 37–40 psi on fine gravel and 32–35 psi on coarse gravel. Silca's model is designed around rolling resistance minimisation, which keeps pressure relatively high on gravel.
SRAM's compliance model produces approximately 33–36 psi on fine gravel and 28–31 psi on coarse gravel. SRAM weights control and comfort more strongly, pushing towards lower pressure.
Pirelli lands between the two: approximately 35–38 psi on fine gravel and 31–34 psi on coarse gravel.
The consensus (average) typically comes out around 34–37 psi fine gravel and 30–33 psi coarse gravel for this reference setup. These are reliable starting points that balance all three models' strengths.
Tubeless makes a bigger difference on gravel than on road
The 3–5 psi tubeless reduction that calculators apply matters more on gravel than on road, because you're already working in a pressure range where small changes have large perceptual effects.
At 35 psi on a 40 mm tyre, a 3 psi reduction to 32 psi is nearly a 10% pressure change. At 80 psi on a 28 mm road tyre, the same 3 psi reduction is only 4%. The gravel tyre at lower pressure provides significantly more compliance, more contact patch area, and more traction — the tubeless reduction is fully felt and genuinely beneficial.
For tubeless gravel setups, always enable the tubeless toggle in the calculator. Running a tubeless gravel tyre at tubed pressures wastes the primary benefit of tubeless conversion. The combination of lower pressure and no pinch-flat risk is the entire value proposition of gravel tubeless.
One practical note: tubeless gravel setups tend to lose pressure faster than road tubeless, partly because gravel riding involves more tyre deformation (which can temporarily open micro-gaps in the bead seat). Check pressure before every gravel ride, not just before long ones.