2009 3400RL (approx weight 13,000), 2005 Duramax with 5" exhaust, running in tow mode, with cruise control set, four people onboard, on basically level ground at about 65 to 70 mph, I get from 9.5 to 10... Thats with all tires at there cold rating (80 psi) except the front truck tires, they are...