So again, RAM/Cummins is working on a new setup that moves all smog treatment post engine. So it appears that mileage will be similar to a deleted truck right from the manufacture. And we all hear above the mileage deleted vehicle get. BIL 2008 RAM 2x4 gets over 25 MPG on the highway. 14 to 14.5 towing a 37' 5th wheel.
Money spent for this seems to return better bang for the buck than what more expensive 8 and 10 speed trannies will provide.