I have a 03 Ram 2500 with the Cummins Diesel 6 speed straight drive and I am lucky to get 25 to 26 mpg driving 55 mph on the interstate with no head wind and with nothing hooked to it. When I pull my 39' 5th wheel I normally see 12 to 14 mpg if I drive it around 55 to 60 mph on the interstate. This driving is mostly flat country. I have not driven much in the mountains yet. I guess that I am doing something wrong or I would be getting higher mpg.
Freddie & Linda
2015 Forest River Sierra 330RLS
2003 Dodge Ram 2500 4X4 Cummins Diesel
Days camped for 2016 - 46
Days Planned for 2017 - 118