I used to run mine at @42-45PSI.
I just ordered some General Grabber ATX tires yesterday ($100 rebate from General if purchased before the end of October) in the same 275-65R20 size and I'll probably run them at about the same pressure (on the stock Ram wheels).
On the pressure, I usually keep an eye on the tread wear across the tire's width to dictate how must pressure to run. Since I sometimes drive my Ram like a sports car, if you run too low of pressure, the tire will roll-under in the harder turns and you will lose the tire's corners and maybe even some of the side tread aggressiveness if you drive it like a Lambo. This is not a good thing for off road use since these areas really help with traction when the poop gets deep. So, if you notice some high corner wear, running the pressure up a couple/few PSI higher will keep the tire from rolling under and it will put the wear more in the middle of the tread.
Likewise, if you notice the center of the tire is getting "thin" compared to the remaining aggressive corners and edges, then reduce the pressure slightly to let the edges do more work and get more contact with the road.
On fuel economy, unless you recalibrate the speedo/ECU to compensate for the taller tires, your in-truck computer will tell you the new tires have cost you about a 10%-15% loss in fuel economy. This number is incorrect due to taller tires making fewer revolutions per mile, and since the truck is eating the same amount of fuel to travel "X" distance with the tires doing fewer revolutions per mile, the computer calculates your mileage incorrectly as a significant reduction in efficiency. I have confirmed this calculation error with a hand-held GPS unit and @ a 200 mile trip on the highway, by topping off the tank and resetting the truck computer just before entering the highway and then topping off the tank and comparing the actual amount to fuel used over the distance along with the GPS readings and the truck's onboard computer calculations. The faster you drive, the worst the speedo error and mileage loss will seem to be. Even as I was driving along with the cruise set, the GPS gave me the correct ground speed and it was different than what the digital speedo was showing.