Talking bout MPG my dads 6.4 gets 22-26MPG and hasnt had a problem, its just which you get, some are bad with problems and some arent, safe bet is getting a 7.3 i get 21MPG out of my 99 7.3 AND its reliable ,but on the other hand it takes money to get serious power from a 7.3 whereas the 6.4 you...