First thing you need on a old Challenger to get better mileage is an overdrive system of some kind. Tremec 5-speed or an overdrive system on the 727. Then a nice fuel injection system for the 340 with an MSD and high energy ignition. Ill bet the mileage would increase by a nice amount. But these cars will be driven hard and not babied so gas mileage is expected to suffer. I believe with the old set-up I was in the 15 MPG range.