For a 75 kg rider on 32 mm road tyres, SRAM comes out ~2 psi lower than Silca. On 40 mm gravel, that gap grows to 4–5 psi. The difference isn't error — it's two genuinely different philosophies about what optimal pressure optimises for.
The two underlying models
Silca's calculator is built on a surface impedance model. The core insight is that every surface has a measurable resistance coefficient — how hard the tyre has to work to deform over the texture. Smooth tarmac has low impedance; coarse gravel has high impedance. Pressure is calculated to keep rolling resistance at its minimum given that impedance value, plus your system weight and tyre volume.
The result is a system that's very sensitive to surface type. Silca differentiates strongly between rough and smooth tarmac, and between fine and coarse gravel. It also gives a large front/rear asymmetry, with the rear significantly higher to carry the greater share of load with minimal rolling resistance.
SRAM's AXS model takes a weight-forward approach. It starts from the same physics — weight plus width drives pressure — but its surface adjustments are smaller and its safety margin is different. SRAM's model was calibrated against real-world compliance testing, where rider feedback on control and comfort was weighted alongside power meter rolling resistance data. The result is a model that prioritises ride compliance over rolling resistance minimisation. On the same inputs, SRAM consistently lands 1–3 psi lower.
Where they converge and where they split
On narrow road tyres (23–25 mm) with a lightweight rider, SRAM and Silca are within 1 psi of each other. The impedance model and the compliance model make the same call when the tyre volume is low and the speed is high — minimum rolling resistance and adequate compliance coincide.
The divergence grows with tyre width and surface roughness. At 32 mm on road, you'll see roughly 2 psi between them. At 40 mm on gravel, the spread opens to 4–6 psi. At 50 mm or wider on rough gravel or hardpack MTB, they can disagree by 8–10 psi.
The reason is simple: Silca's impedance model has a large correction factor for rough surfaces that keeps pressure high to maintain momentum. SRAM's compliance model has a smaller correction because their data showed riders were happier (and faster over mixed terrain) with lower pressure than the impedance model recommended.
Why SRAM includes the hookless cap
SRAM adds one more constraint that Silca doesn't: a maximum pressure cap of 73 psi for hookless rims. This is the ETRTO safety limit for tubeless-ready hookless rim profiles.
For most gravel and MTB riders, 73 psi is never a binding constraint. But for lightweight road cyclists on 25 mm tyres with hookless carbon wheels, the base formula wants to go higher — and SRAM hard-caps the output. If you switch from hooked to hookless rims in the calculator and see the pressure stay the same or drop, this cap is the reason.
Silca doesn't apply this cap because their calculator was primarily designed around hooked rim geometry. When using Silca with hookless rims, it's worth manually cross-checking against the ETRTO 73 psi limit for road use.
Which one to trust
On smooth to moderately rough road surfaces, use Silca if you want to minimise watts and SRAM if you want a slightly more compliant, forgiving feel. The difference is real but small — most riders couldn't feel 2 psi in a blind test on normal road.
On gravel and rough surfaces, the gap is large enough to matter. SRAM's lower recommendation will feel more controlled on variable terrain; Silca's higher recommendation rolls faster on the smooth sections and slows on the rough. The consensus number — the average of both — is a practical compromise.
If you frequently ride both smooth and rough sections in the same ride, the consensus value is your best starting point. Run it for two rides, then adjust 2 psi in either direction based on feel.