The proper tire inflation will be listed on the data plate on the door of the truck. Go with that and not with what it listed on the tire.
Also, for a smoother ride try changing out the tires. I use to keep 6 ply tires on my truck, and it rode like a TRUCK! Now I use 4 ply for a smoother ride. Yes, I have had a few more nail holes that needed patching but I won't go back to the 6 ply.
Just curious, have you priced those 31" critters!
That is a good choice for a truck.. If you can get that type of MPG, you are doing better than most. My neighbor has the same 2008 truck and he is getting right at 20 mpg on the highway. City it is down around 15 mpg.