[TAKPAK]

Johnpjpa; we have a 2007 3400RL, towed with a 2008 GMC K3500 (four wheel drive) rig. My policy for driving has been thus......
The Duramax's peak torque is at 1600 RPM. I try to keep it around that range. What I've found is, if on level road (freeway) and no headwind, I run with the Allison in "normal" (i.e. not in tow/haul mode) and we are running about 62 MPH (6th gear). If we run into headwinds or a steep hill, I activate tow/haul, which keeps the speed the same, but the RPM goes up to around 2000 RPM (now in 5th gear). Worst mileage ever......10.2 MPG, going over a mountain. Best milage ever......13.5, running on the freeway.