@Hot Rod Lincoln the 500cid Cadillac V8 in my C60 school bus in front of an SM420, gets 6-7mpg depending on how heavy my foot is. The rig weighs about 15,000lbs. However I've now got the 5.57 rear gear with which to upgrade from the 6.50, and I think that will help my mileage some. The big V8 has no trouble with mountains, and it also holds me back real nicely going back down the hills. I barely have to touch my brakes.
