I have a '10 2500 4x4 with a hemi, it has something attached to it every weekend. Most notably my car. 2 weeks ago was a 400 mile round trip from Seattle to Portland, pretty flat drive 14mpg, hand calculated.

This weekend I went from Seattle to Boise and back, 1000 mile round trip over lots of mountain passes and I got 11, hand calculated.

Hemi does fine on even the steepest of the pulls, it will down shift, find a speed to keep the RPM's about 3000-4000 and it will go up anything I've found as fast as you want to go.

Would I like to have a Cummins sure, can I justify the added cost, no. I've done these exact same drives in a comparable 04 cummins, and the MPG difference is negligble once fuel costs come into play. The newer diesels don't get anywhere near what the older trucks did.