My 08 2500 duramax averaged 14 mpg with mixed city and highway driving.. tuned and deleted it averaged 23 mpg driving in the same conditions, but loaded pulling a gooseneck trailer with mini-ex i still got around 17-18 mpg. My 12 valve cummins i drive now averages in the 20s and it seems about the same loaded or empty. If you plan on pulling anything heavy, the cummins gets my vote, not that the duramax wont do it but i prefer the power curve of the cummins for pulling heavy
