With my diesel, best mileage is at 64MPH, running 1700 RPM. I bet if I get up to 65-70 it would drop alot. Now, mine is a Cummins, and in flat tow, on cruise, Im getting right about 11.5 mpg. I don't think that would drop to half or so at 70, but I don't run at that speed, because I have ST tires, and they will self destruct at 65.
As far as deletes etc go, a friend of mine has a Ford F250 diesel (2008). He did the DPF delete and a tune. He was getting 10 to 11, and now he's at 11-12, but more throttle responsiveness.
Granted, neither are Duramax's but it's a data point.
As far as 15MPG towing a 40', I'll call BS. Don't see that happening. Nothing short of me sitting in the right hand seat for a 1000 mile trip will convince me otherwise. Just my thoughts.
Mike
