I've had this truck for about a month. It has a 450 Cummings, 10 speed. Most of the loads we haul are over 40,000. When I first got it I was getting 6.8 to 7.2 mpg. All of a sudden I am down to 6.1 to 6.3. It was serviced when I got it, and I have put approximately 9,000 miles on it. Any thoughts??