Whats the correct tyre pressure

IMO, you could do worse than try 38-40 for the blacktop, 28-30 for the gravel (with slower, careful speeds of course) and around 16 - 18 in the sand...with that width of tread, they should be real demons in the sand, on a Triton.

I'd pretty much agree with Banjo, but given the vehicle weight and the tyres used, I'd suggest
32-35 on bitumen, 23-25 on gravel, and air down to whatever on sand.

Cheers
Phil

On the road I run my Desert Dullers at 40-45psi. Less for gravel, and about 12-18psi for sand.

Mate I bet you're non the wiser after all the answers you've received. May I suggest that you try the 4psi trick to get a reasonable pressure. This only applies to travel on bitumen (and I guess really good quality dirt roads). I suggest you use 32psi as the starting pressure. Then go for a decent drive - long enough for the tyres to get to normal operating temperature. Then stop and take the pressure readings. If the readings are about 4psi more than when you started then you had the correct starting pressure. If you have more than 4psi above starting pressure then you didn't have enough to begin with so add a bit and try again. If you have less than a 4psi increase it means the starting pressure was too high so let a bit out. In time you'll figure out how much you need when empty and loaded. Obviously ambient temperatures will make a difference but at least I hope to have given you a means to work it out for yourself. This method, I believe, comes from Pirelli, but I may be wrong.
As others have said you'll need to adjust when off the bitumen to suit conditions.