I believe the air down pressure depends upon your normal psi. Different size tires have different psi ratings. Also, it depends on the terrain and how fast you will be driving. For what its worth, I'm still running the stock 245/75/16 tires that came with the SMB. I run them at 80psi on the highway. I've aired them down to 35 psi off road, but never went faster than 25 - 30 mph that way.
I use a Viair portable compressor to the air them back up. It has worked great and is pretty comparable in peformance to the onboard compressors. I have a prior post about this in the 4x4 forum.
2007 Ford RB Diesel SMB 4x4 Pueblo Gold; Custom configuration (aisle layout); PIAA 580 driving lights; Picked up on Oct 19, 2007.