I can't compare the 6.7 never had one but I had a 5.3L and pulled my 6000lb camper with it to Nova Scotia and got 7-8mpg at the best of times and that was driving 60-65mph. So I traded it in on a new 3.0L and took my camper out west to Wyoming then down to I-10 and home. If i watched my speed I'd get 13-14 MPG but most of the time I just drove the posted speed and still got better than the gaser at 10-12mpg and that was doing 75 most of the time. And when not pulling the trailer I'd get 29-30mpg. For me the choice was easy, just depends on what you want your vehicle to do.