I've owned quite a few 1500s with stock 5.7s and various engine swaps and the 6.4 SRT8 engine swapped 2011 got the best MPG of them all.

Also people should not be surprised the 6 cylinder gets worse mpg, it was built because some stupid engineer convinced em they needed new technology to get past emmisions (Chevy and ford have no problem getting pushrod engines past emmisions). And on top of worse MPG it needs premium fuel vs the hemi being just fine with low grade.