My 3.9 4x4 auto gets a consistent 13-14 around town in the summer, and around 10 in the winter. My old carbed 360 4bbl 80 Dodge 4x4 got 8-9 all the time. I was and am thrilled with 10-14 mpg. Its all in perspective.
On the highway, for the first few miles, it seems like I really have to push the truck to get up to 65, but after a few miles, it loosens up and runs. After a 1/2 hour of highway driving I need to watch my speed, it runs easily at 70+. I'm getting 17 + on the highway. I really never have driven for great gas mileage, I seem to be in a hurry all the time, or maybe its my lead foot..

Don't blame the truck because of a bad thermostat, or short drives, and don't make a judgement call on one tank of fuel. Gene