While yes your 6.7L Cummins in a big truck is only averaging 10.5mpg stock...I`d imagine something like a 5.0L Cummins to walk circles around a 5.7L Hemi. I took a few Hemi`s out for a test when I was shopping for my truck. The overhead display never got above 12mpg around town. No matter how easy I went on it. Cruising on the highway it never got higher than 17mpg.

Keep in mind the Hemi`s I took for a test were all 3rd gens...not sure if the 4th gens get anywhere near the claimed economy. I know the 3rd gens sure didnt.

 

I'm just not sold on a 5.0 being able to get good mileage when pushing around a 6k+ truck. Basically you'll take your truck, reduce displacement by .9 L, then cripple it with emissions controls, and shave a few hundred pounds of weight. Certainly it won't be lighter than a Hemi, and most likely if the 5.0 is built anywhere near as beefy as the 5.9/6.7 it won't even be all that much lighter than the ~7500 pounds mine is with a full tank of gas and me in it.

In areas where emissions aren't tested/inspected, a light duty diesel would be awesome, but like I said, I just don't see it happening any time soon.