I truely believe (and studies have proven) that driving style can vary mpg up to 30%. My wife consistently gets less mpg in the same vehicle than I do. She's on the gas until she needs to brake, and I coast a lot more and look further down the road, she takes off more aggressive than I do too. That's just a couple minor examples of how a person's driving affects mpg. I'm pretty easy on my stuff when I can be, but -- when I need it to perform, I let it rev and push it.
I'd believe that a 2 (on the low end) and 4 (on the high) is about the mpg difference between the 6.4L Hemi and the 6.7L Cummins. But, remember that the Cummins makes 370 - 385hp and 800 - 900tq and the Hemi makes 410 hp and 430tq. That's a big tq delta.
