Don't know about the 6.7 but I'm happy with my 5.9 w/210,000 miles. Here's a picture of my overhead showing my average mpg after a 200+ mile round trip cruse @~68mph:
https://www.dieseltruckresource.com/...0&d=1247691528 Hand calculated it came out to 20.6mpg.

I've got another one on my phone that I haven't downloaded yet showing my overhead with a reading of 23.5mpg for a completely different 300mile trip with the cruse set to 65mph. That tank hand calculated out to 21.1mpg.

Neither of these trips were completely "ideal" and I wasn't on the open freeway... just single lane Texas FM roads with 70mph speed limits. There was hills, light wind, small town traffic lights, and some idling. I believe this truck could see 22mpg if it was 100% Interstate freeways (cross state or state-to-state road trips)

 

I regularly see 19 to 19.5 miles per gallon, highway and interstate speeds, unloaded, with all emissions gone, turbo back that is.

Under hood deletes will be done in the future, upon procurement of a Smarty.

Oh, and thats hand calc.

 

Just put 1100 miles going from New York to Maine & back with my stock 08 3500 mega cab 4x4 6 spd auto 6.7 diesel & got 15.5 mpg. I did have it loaded up with family, gear, bike rack on back & cooler/ fishing rods in front. Only have 4500 miles on it so hopefully the mpg will improve after the motor loosens up.

 

my mega 2x2 with 6.7, 3:73 posi, auto, gets around 11 towing 10-11,000 lbs averaging 63 mph. empty i get 18-22 depending on speed and terrain.

 

I'll give you my 2 cents worth. This is my first diesel and I love it. I pull a 34' fifth wheel camper and get 16 MPG cruising at 65 - 68 MPH. The worst I've gotten is 14.5 in the hills of east TN. When I had my 4 horse gooseneck trailer fully loaded with 4 horses and gear I got 17 MPG. This is my daily driver and around town and a bit of interstate driving to work I get 19.5 - 21, and that's not going light on the pedal (I like to play!).

Bottom line is this, I love it!

 

I could get about 18 with my 98 GMC at best and regularily get 20 (combined hwy/city) in my 5.9 Cummins. Towing, the GMC was 12 at best (as low as 6 with a serious headwind) and usually 16 with the CTD. No comparison IMO.

I B-trained a TT and boat with each and yank my bobcat once in a while with the CTD.