I HATE JELLY BEAN CARS
But it is possible to get OK mileage from a performance engine
Got 17 mpg out of my 72 340 4 speed Challenger with 3.55 gears and 60's all around
can get 14 MPG out of my Dart with a 71 340, comp cam, air gap intake and PM ignition, 3.23 gears, 650 Thunder series...