I've got a 6.7 in a 2013 model. So far so good. On the interstate at 75mph, I'm getting 18.5 mpg. Lots of stop and go it drops to about 16.5 on average. I haven't hooked any heavy trailers to it yet, but I'm very optimistic that it's going to have plenty of power to full my 34' fifth wheel.
It is a little scary to know that you can't even change or check the belts on the engine without taking the cab of the truck off. They have it packed in there tight.
It's not that bad but you can't do any head work without pulling the cab. Belts are still accessible- kinda
