Dunno about the 6. 7 C/C, but my initial fuel usage tracking has been a little more encouraging with my brand new 3500 SLT quad cab, 4x4, 6. 7 68RFE.
I drove my '97 to Glenwood Springs, unloaded all my stuff (a ton's worth of power tools and parts bins) into the back of my new 3500, and headed up to Aspen to check on one of my crews. The trip to Aspen (about 55 miles from the dealership) was into a headwind, and included 22 stop lights, (I managed to hit about half cycling to red).
I put the cruise control on 60 mph, and just let the truck be pushed side-to-side by the strong winds. Aspen lies at an elevation of 8000 ft, and Glenwood Springs is at 5700 ft, so the trip from the dealership was uphill all the way. I watched the overhead console, and the truck managed 14. 3 mpg average over the trip up the long grade.
The return trip to El Jebel and home is about 25 miles, and is downhill all the way, so, naturally, the truck glided most of the way, except for the last two miles which is a steep, uphill grade (1000 feet in about . 6 mile), and the average -- including the return trip home -- brought the fuel-use number up to exactly 17 mpg at the El Jebel turn-off, which is half-way back to Glenwood Springs.
The steep ascent up the mountain to our house overlooking the valley knocked the average down to 16. 5, but I was still pretty impressed by this first 85 mile trial.
I figure that if I'd let the truck coast the rest of the way back to Glenwood, the fuel usage would've been mid to high 17s, which I think is pretty awesome for a new, heavy truck with a very-tight drive train, and on a very windy day. Also, we're still burning "winter fuel" in this area; it still drops down in the low twenties at night, so I'm sure the summer fuel story will offer more encouragement still.
I don't know how accurate these computers are --- could be off, significantly, but I'm pretty sure that this drive-train (engine/transmission combo) will produce significantly better results than those indicated by "Diesel Power".
Personally, I'd be tickled to death to see an average of 16-17 mpg (loaded) on this big rig, and, after she's fully broken in, I think that's a reasonable expectation.