I don't know about 80 MPH, but on the flatlands with cruise control at 70 MPH I see 20 MPG in my '03 measured at the pump. Now towing, that's another thing. At 65 MPH pulling my massive 37' triple axle, 14,500 lb loaded, toyhauler I averaged 10. 8 MPG from Oregon to Glamis, CA and back. About 2,000 miles.
My '03 is the best of the three previous CTD's I've owned. It doesn't get quite the fuell economy of my old 12 valves, but it sure rides and performs better.
Sometimes I wonder why the large varience between trucks. Mine has the six speed and has not been re-flashed. Count me happy.