I have an 08 6.7 and the best milage I have got with mine is 19 according to the overhead but that was just one time. I average around 14-16 hand caculated. I have heard some people that have had problems with there 6.7 but mine has been flawless, I love it!!
brian I’ll bet the only issues you’ve heard of were smog related. People who have got rid of the DPF, EGR, and CAT, then added a tuner to remove the codes have no problems, and some are reporting up to 25 MPG!
I have a 2007 6.7L (built February 2007) cab and chassis, only issue I’ve had it wouldn’t down shift into passing gear, trip back to the dealer, 2-minutes for a new flash, perfect!