this video is good info about how much to air down for different conditions. maybe this should be a sticky for the tires and wheels section

http://video.google.com/videoplay?docid=...earch&plindex=2

This is what the video recommends. It's a 10 min video and takes a while to load if you've got a slow connection.

Terrain (Percent to lower tire pressure)

Sealed surface (100) -No reduction
Gravel (10-15)
Rocks (10-15)
Mud (25)
Sand (50)
Slippery/Clay (20)

Not sure if that's what I would do. Here's why.

Assume normal air pressure of 35psi.

Gravel / Dirt: 25psi (total reduction of 29%)
Moab: 18psi (total reduction of 49%)
Snow: 10psi (total reduction of 71%)
Racing: 50psi (total increase of 43%)

NOTE: I would only run these pressures if using an aftermarket tire with superior sidewall reinforcing. The deflation may cause less clearance under diff, but this is not a big deal... dropping the air pressure will grant you traction that is more valuable than clearance issues (pick a better line!)