30 mpg is very optimistic. My Dart got around 18 mpg in city driving before the latest round of mods, and it's pretty rare to hear of one going over 25 mpg. Maybe if you went hog wild lightening the car and went for EFI...
Have a good friend who bought a rust-free, bone stock slant 6 Duster out of Fl and brought it to MI (bad mistake) to use as his daily driver. He said in normal everyday driving it would get around 20MPG. Just for giggles and kicks he brought it to the Nationals with a goal to see if he could get some decent mileage out of it on a long trip. By driving a pretty steady 55, he was able to get 27MPG out of it. Again, it was a bone stock 74 Duster with a 904, PS and AC (AC not used on the trip). Within 3 years of using it as a daily driver, it was a rusted out hulk! Gotta love them MI winters!