I've got the 5.9 2006 4wd quad cab & have put all 175k on it. Still runs just like the day I bought it. I'll get 21-22 sometimes 23 on interstate 60-65 hit 70 & it starts dropping fast. It'll tell me I'm get 25 mpg but caluculations tell me 22 or so. I have a LARGE AFE Filter, no cat & 5" magnaflow exhaust from turbo back & a programmer. Each of these items jumped my mpg by 2-3 mpg. Ticket with a diesel is to let it breathe! What air is going in must go out! Sitting in my truck at idle I can hear the air going thru the intake.
The 5.9 is a stud. I know there was a few bugs with the 6.7 on the polution crap. But don't think it was anything major