fuel problem with a 1972 dodge dart

I have a 72 Dart Swinger with a 318, 2bbl and a A-727 Torqueflite transmission. My family has 3 Mopars my Dart a 65 Coronet 500 (hemispherical heads 318 , 2bbl) and a 67 Plymouth Fury 2 (poly 318 , 2bbl) . Im getting 7.8MPG and i cant understand why the other 318's get way better fuel economy. Can anyone explain why? Im a bit new to owning a Mopar so any help is greatly appreciated. Thanks in advance