Super six set up with low comp

Just as a point of reference, my mon's 1980 Super Six Aspen wth auto got 22 mpg on the interstate and was fine for mpg in around-town driving; it's mileage was not any lower than the 5 other family Darts that preceeded it, except for the Dart Lite obviously. I guess you could get lower mileage if you put your foot in it, like anything.