Fact: The 1976 Feather Duster and Dart Sport Light with the 255/883OD were the first US cars to get over 31mpg.
My 75 Dart Sport with an auto got 27-28 highway all day long. My 79 B200 w/ 318 883OD and 355 gears got 25 highway.
If your getting under 20 in an A-body then something is wrong.

72 Satellite Sebring Plus 440, 72 Dart 5.9 4-spd, 68 Valiant, 73 W200, 78 D100 sb, 78 D200, 98 DAKOTA, .
Moparmarks Parts & Restorations
Desert Mopar Metal
Grand Jct. CO
970-261-7039
http://moparmark.com/
motormangj@gmail.com