Gday Gary
I don't have any experience with 20" tyres, but I take my 255/55/19's down to 18 psi without any issue. I think you could go at least to 20 psi, and lower in an emergency.
On my last beach trip I didn't drop psi in the tyres and it got through easy (not towing anything). A 3.0 D4 should be similar. Check the conditions first, and remember momentum (not high speed) makes crossing to the hard stuff much easier.
Good luck.


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) turbo, p/steer, RFSV front axle/trutrack, HF, gullwing windows, double jerrys etc.


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