I have a slant 6 dart automatic and its only getting around 16 mpg , mostly interstate driving some on the back roads. is this average? would a 4 speed overdirve help verymuch? it is winding pretty tight on the interstate but i am not sure of what the gears are in it. and i dont have a tach in it either but it screams right along.

just looking to get some better mileage. i got 16 mpg with my 318 cuda!