I just replaced my 2001 7.3 powerstroke with a new 2016 6.7 powerstroke 4X4 I know the 6.7 won't get the mileage that my old truck got but once it's broke in what kind of mileage are you 6.7 owners ...
I like to keep up with the mileage for a number of reasons. One of the most important is to make sure the rig is operating normally. If my mpgs were to drop noticeably, I'd be looking for such things as a brake caliper hanging up, an injector going bad, the tranny slipping, etc and etc.
I also like to know how headwinds affect my mileage, and my speed, and the hills, and the etc and etc.