For a 76 kg rider on 40 mm gravel tires, three pro calculators converge on 39–42 psi. The spread is 6%. Below is why they disagree — and which one to trust on broken Class 3.
Tire pressure is the single setting on your bike that costs nothing to change and gives you the most. Drop 4 psi on rough gravel and the bike disappears under you; add 4 on smooth chipseal and you find a free watt. The hard part is knowing what 4 psi to add to what number — and that's where the three big calculators stop agreeing.
This guide walks you through how SRAM, Silca and Pirelli each model the problem, where they line up, and where you should override them. The math is in the appendix; the field notes are in the body.
1. The three variables that actually matter
Every reputable calculator takes the same three inputs: rider + bike weight, measured tire width, and a coarse surface grade. The rest is brand opinion.
2. Why SRAM is the most conservative
SRAM's model weights compliance more than rolling resistance. The result: on the same inputs, it consistently lands 1–2 psi lower than Silca. That's a feature, not a bug — SRAM's data comes from broken European gravel, where grip beats glide.
3. When to trust the consensus, when to override
On a known surface, the consensus number is correct ~85% of the time. The other 15% — broken pavement, wet roots, hot rims on a long descent — needs a human override. We cover each in turn below.