It's interesting to hear a dealers perspective on this. We recently replaced my 2008 F150 5.4. Our leaser was pushing us hard for the ecodiesel Ram, because of the fuel consumption. We actually ordered one. Then the next day literally a truck fell in our laps due to a death in town. (unfortunate yes, but someone was going to buy it). We ended up with A 2013 F150 5.0.
The interesting thing is that on the highway it gets mid to low 11's. So when we did the math and actually calculated the fuel used. It is exactly half of what was being used before. BUT, the difference is the RAM equally outfitted cost 10k more. Now we pay fleet for our trucks so the numbers are a little different, but the idea is the same. So in the lifetime of the truck with me as a driver, It would only break even on cost after 250,000km. Similar to the Eco-boost, when they first came out. It seams it is strictly a pay now or pay later scenario. Considering that we will probably deal this one at 150... it would have never paid off.
Of course fuel consumption is different for everyone and every vehicle, you mileage may vary. Although I really enjoy my new(to me) f150, I was really excited to try the ram with the baby diesel. Maybe next time