The ATs are pretty popular with desert prerunner vehicles, and have excellent sand traction.
There are better options for the things 80s usually do (rocks and mud), even in the at catagory, though the BFGs do have a very long tread life.
Well for the best milage that would be whatever the max PSI is on the tire sidewall (and maybe a couple more on top of that).
This will also result in dangerous handling, reduced stability, reduced traction, reduced comfort, and general... dangerousness.