I went from a history of two cummins diesels to the 6.4L Hemi.
Mountain towing is not a problem with this engine. The rv I tow is a 37' 5th wheel that has 3 slides and weighs about 11000 to 11500#, depending on how much we pack. I live in the flat lands but always tow to the rockies. In the flats, we fight headwind. In the mountains we climb and descend grades.
The 6.4L Hemi and previous 5.9L Cummins both climb the same grades at about the same speeds. The difference is that the Cummins did it in 3rd (4 speed auto) at about 2200 - 2400 RPM, and the 6.4L Hemi uses 2nd gear (6 speed auto) at ~5000 RPM.
Towing mpg with the Cummins was 10-12, and the Hemi gets 7-9 mpg hauling the same RV over the same route. The diesel had 3.73 gears, the Hemi has 4.10s.
On the mpg front, an important thing to note is that the 6.4L Hemi is able to switch into 4 cylinder mode when running empty, the 5.7L in the 2500 doesn't have that ability as it is not equipped with MDS. RAM engineers say the 6.4L Hemi will therefore get better mpg than the 5.7L (
Video).
Also, the 6.4L hemi has only slightly more power than the 5.7L, but it makes peak torque 800 - 1000 RPM lower than the 5.7L, GMs 6.0L Vortec, and the Ford 6.2L gas V8. That is what allows MDS to be used, and is great for towing. They accomplish this by using variable length intake runners. Good low end, then they change to add more top end power. If you listen carefully you can hear it happen when you floor it and the rpm climbs.
Typically, my truck will haul my RV at 2200 RPM in 5th gear and hold there down the highway when in flat terrain.
Here's a video of my truck towing my RV.
Link
