Maltese, I have basically the same truck you do. I was very disappointed with milage early on, 13-14 rolling off the lot, especially when I bought it trying to improve milage over the Ford V10 I traded in - yeah, noone told me the 10 in V10 referred to the mpgs! . Anyway, give it time. Now with over 35K on the clock I get an average of 20.2 with the brew and 19.5 without. (rolling empty, 160-170 miles daily - 1/2 at 75 and 1/2 at 65). And I don't baby it, in fact it seems to like me better when I crack the whip just a little.

 

Not to just jump in, Cummins break in is around 8 - 10K miles for the average owner. Less if you work it like a dog. To save on the tranny, tow with the tow/haul button on, or OD off depending on the year. Saves the tranny. The more you work it, the more you want to replace the tranny fluid. Even if you just drop the pan every 15K and change the filter every 30K, it will make it last.