"Different Strokes for Different Folks"! Well that pretty much describes the debate over tyre pressures! Every vehicle is different; it's weight including distribution, driving styles and of course what type of rubber is on the rims.
Different people have different ideas, so we can only tell you about our own experience, and we definitely reckon, "lower and slower" is the way to go.
Higher pressures on the dirt don't work. Although your sidewalls are more straight which in turn, in theory, will protect them, this doesn't stop tread damage from sharp rocks, and that's why most flats occur.
If you reduce your pressures on the dirt, this makes the tyre more flexible and allows it to better "roll over" sharp stones and the like, particularly on loose gravel roads.
Reducing pressures means you get a much bigger footprint on the road too, and this can be a real positive on the dirt, cos this will give you much better traction, especially into corners, not to mention a much better ride over those inevitable corrugations. Higher pressures leave you with a smaller footprint, and so can't give you the same stability as the lower pressures.
The downside to lower pressures, is the risk of the tyres overheating due to the larger footprint on the road, but that's easily sorted by slowing down from highway speeds, and let's be honest you probably shouldn't be at those speeds anyway on the dirt.
On the sand you need an even bigger footprint, so that's why we lower them even further - the bigger the footprint, the less chance of getting bogged! Never go too low though, cos that increases the risk of the tyre rolling off the rim!
So - what do we run in our fully laden 80 Series Cruiser (as a guideline)?
- On the bitumen about 34 to 36 psi,
- On the dirt about 26 psi,
- On the sand about 18 psi, though big sand dunes we'll go down as low as 11psi (even lower to get ourselves unbogged)!
Smaller, less heavy vehicles of course, won't need to go as low.