I've already posted this a couple of times, but here it is; the best fuel economy I've ever seen in a diesel pickup. A couple of months ago I got MPG crazy and came across these articles in my research.
1997 Dodge Ram 3500 Dualie Cummins - Diesel Power Magazine
That's a 12V Cummins dually that gets 30mpg.
2000 Ford F350 Power Stroke Diesel Truck Mileage - Diesel Power Magazine
That's a Powerstroke 7.3L F350 2WD that gets 26mpg.
I've not seen a test conducted on a Duramax, but seeing as how my '03 can get 23mpg if I drive easy in warm weather, I have no reason to believe that a duramax couldn't surpass 26mpg with similar modifications. If I had a 2wd regular cab to use as a test mule, I'd try it out. Also with EFI live, I bet you could make a fancy tune to sip fuel below 2000rpm right at 60mph