The earlier 6.0 engines had a few problems (03-04). It really is a hit and miss with the earlier years of that motor. The 05+ are great. We have an 06 F350 6.0, and we haul a 40ft trailer (18,000lbs!) When pulling the trailer, we average 16-18mpg. When not towing, we get 21mpg give or take a bit. (Freeway miles). It's a great truck, and we've never had an issue with it. Good luck =) You'll love the SD!