Just bought my 2004 3/4HD Duramax longbox and drove from Colorado to California. Using the DIC I got high 18s driving between work and home the first few days prior to trip. Speeds didn't exceed 65 and had stop and go driving some of the way. Once we hit the interstate on our trip we only saw ~17 mpg, lots of hilly grades getting into Wyoming and high head winds. Once we started west bound at 75mph on cruise I got 18.7 mpg, I checked manually and DIC was within .4 mpg of actual. Now once I let the wife drive she set cruise at 80 mph and mpg dropped down to 17.8 mpg.
I filled in Reno and drove up and over Donner Pass and down into Chico speed limit was 65mph on freeway and ended up driving many back roads at 35 - 45 mph, on that tank I got 20.5 mpg. Truck is completely stock and has 26K miles on her. There were 3 adults and two children on board.

Running with my 8.5 foot truck camper (pop-up style) @1600lbs +gear=total of 1800lbs~ and a ATV trailer with two ATVs added another 2200lbs, running back roads and in the mountains taking on several passes and using tow haul mode and keeping my speed no higher than 65mph, my mileage runs about 13.8 mpg.
WOW I love that trans! Makes hills disappear and control in turns great. Oh stopping was much better with trans downshifting for me!