I just took a 364.0 mile trip from Mandeville, La to Houston Texas. I used 20.521 gallons and managed 17.737 mpg.
06 5.4 3.55 gears, 6.5 box. 100% Stock. Used the a/c (defrost) for about the first 1/3 of the trip. I did not stop along the way, ran into some moderate traffic. With the exception of passing, I maintained 75 mph whenever I could. Average speed for the whole trip from the gas station in mandeville to the hotel in the copperfield area was 66.18mph due to some traffic lights and some low speed zones/construction in addition to a detour on 610 where I had to make a near stop to make a u-turn. I did floorboard it briefly on my detour, but drove moderately most of the trip (acceleration).

I filled up when I got to the hotel after driving about 2 miles roundtrip to a local store. The gas that I bought was 10% ethanol. I'll find out if that makes a difference on the way home.

I used to get 18.5 mpg in my 7.3 psd for the same trip. I figure adding 4x4 and getting 1 mpg less is not too shabby IMO. I have only 5000 miles on the truck and used regular penzoil 5w20, so she may not be completly broken-in.

Anyway, I was waiting for a while to get these rusults and I wanted to share.