When I see the temps climb over the 1000 degree mark I back off usually dropping a gear... I could easily keep the speed up but I have seen a noticeable increase in mileage by keeping the speed down to 100 kms (63 mph) and taking it easy up the hills. 63 in fifth gear is about 2000 rpms in my Chevy... the sweet spot for the Duramax.
DOWN to 63 MPH??? I'm keeping my Duramax UNDER 63 MPH, usually around 60-62. On level ground, I take it out of tow/haul, and it cruises right at 1600 RPM at 62 MPH, which is the peak torque output for the DM. IF I am in traffic, or approching a hill, either up or down, then I kick in the tow/haul for the shifting and the braking. Otherwise, I cruise at about 62. At that point, I'm averaging between 11 and 12 MPG, with the 3400 behind.
It's like they say, if you want to find out what pulling the 5'er is like, hold a piece of 4X8 plywood flat on into a windstorm. You'll find out real quick......!!!!
