In my view, diesel engines and fuel efficiency follow a simple formula: Run the engine as close to its torque peak, and you'll realize the best fuel efficiency.   We have a 2005 Dodge Ram 3500 Cummins 5.9L, stone stock engine, no "chip" or exhaust modifications, the truck is just as it came from the factory (purchased new in fall of 2004).  We have a friend with a 2004 model, similar, and we've compared fuel mileage for years now.   I've gotten as much as 25 mpg on an unloade