What Peterbilt class 8 big rig truck Gets The BEST MPG?and how do you improve your MPG on your big rig truck?

someone i know wants a peterbilt but he wants it to have good gas mileage so what peterbilt gets the best miles per gallon? and how can you improve your mpg on your current tractor trailer truck? thanks|||the only way to get good gas mileage is to drive at the normal speed limit. or have a new peterbuilt. you really can't save much. or you can try going under the speed limit.