I've made that trip down the interstate towards Bozeman several times with the cruise set at 75 in my duramax pulling my triton aluminum 7x24 wall enclosed and don't ever remember being passed by all these speed demons doing 80 to 85. They must be traveling on the moon where there is no gravity and gallons are twice as big. I know that my truck with a banks kit on it will get anywhere from 8 to 16 mpg on that trip depending on the wind at 75 mph. get real guys. By the way my work truck is a ford so don't think I'm a chevy guy....
Probably because you aren't driving through Nebraska and Wyoming where the speed limit is 75-80. Ive been passed at 80+ before on I80 by other rigs.