From what i've been reading in the 6.7 threads, it seems that they're getting anywhere between 14-18, occasionally a strayer at 19. In the 5.9's, i've heard anywhere between 16-22...it all just depends on what you're going to be doing with it.
I have the 5.9 and as a daily driver, i average about 21 between city and highway driving. Strickly highway and long runs, i've done a bit better with a 22 hand calc, but when I tow the 16' dump trailer, i average between 16-18, depending on if its empty or loaded with crap and I try to drive without putting my foot through the floor.
As far as this thread, the 5.9 is easy to mod, the prices are dropping on the used ones and it gets the job done...no b.s. The 6.7, especially in the early transition (07.5's), many have had mileage issues, CEL's and whatnot and i've read that the 08's have corrected these issues.
So in short, if you're looking for mileage - 5.9
If you're looking for more power and quicker towing, then 6.7
Can't have both, unfortunately. Although, there's some impressive 5.9's mod'ed to the ying yang and they're pushing over 500hp and 1000+ tq.
Hmmm...less expensive 5.9 with a ton of mods to outrun the 6.7? Guess that's a choice you'd have to make too. But I couldn't tell you what the end result would be on mileage.
:ah: