What happens to the footprint and the sidewall as you let air out for sand, one published step at a time. An illustrative model, not test data.
Fig 3Air down↑ ↓ pressure · space presstap gauge: air down or up · tap: pressroad pressure
Tyre, pressed with the same load each timeThe same tyre at road pressureFootprint from below the glass, one outline per step (road pressure dashed)Floor on the gauge: 1,4 barA tyre pressed onto a glass plate with the same load at every step, while a camera under the glass shows its footprint. Each step lets out 0,3 bar, except the last, which stops at the 1,4 bar floor. The footprint gets longer, not much wider, and the sidewall bulges as the tyre flexes more. That longer footprint spreads the weight over more ground, so on soft sand the tyre rides on top instead of ploughing in. The cost: the sidewall flexes more and builds heat, so take speed off with every step, and the tyre is easier to roll off the rim under hard steering. Reinflate before the tar. The floor is a limit, not a target: stop when you have the footprint you need. This is an illustrative model of a generic light-truck tyre, not test data: the flex is exaggerated so it can be seen, and no pressure is shown for your tyres.
The footprint check. Drop one step. Drive a short distance. Look at the tyre. You are looking for the footprint to lengthen and the sidewall to bulge slightly, not for a number.
Published guidance: BFGoodrich Africa. Drop 0,3 bar at a time; do not go below 1,4 bar; at 1,4 bar do not exceed 25 km/h. BFGoodrich also states there is no optimal pressure, as every terrain differs.
This is a starting point, not a specification. Correct pressure depends on your vehicle's actual laden mass on each axle, your specific tyres, the surface and your speed. It does not replace your vehicle and tyre manufacturer's specifications. Reduce speed whenever you reduce pressure, and reinflate before returning to tar.