the 6.4 hemi is a brute, it was great and I loved the truck. I put 6000 miles on it no problems. fuel economy was about the same as the xd 5.6 until about the end of oct. I'm guessing they changed the fuel blend and the air got cooler. my best mpg ever 17.2 all hwy. most of the time I avg. 13-14. I got 10.3 pulling 7000lbs of camper and stuff,driving from Lufkin,tx to Atlanta state park near Texarkana that was both ways figured by hand. the trans is kinda fishy but I was fine with it. it would be the best gasser made if it had ratios similar to the gas xd or heavy duty zf 8spd. cool air and all I am avg. 14.1 in my gas xd, I am ready for summer to see if it gets better. it is actually faster than the hemi and weighs about the same. the trans ratios are so much better but can shift a little funky sometimes. hopefully this will be corrected with software updates. biggest problem was the stiffness of the suspension, although I didn't mind, my wife was pregnant and always thought she was going to have the baby in the floorboard. and she had to mention it would make our baby have shaken baby syndrome so it had to go.