75/76 Dodge Dart /6 average MPG and improvements

The slanty could get decent mileage on flat ground with little load. The 75/76 Darts are the heaviest though, they were pushing close to 3400 lbs toward the end of the A body production because at that point, people started getting interested in a quieter ride, air conditioning, power everything, etc.
The lightest A body would be the 63-66 Convertibles with a manual top.
Oddly, in 1976 as a fuel saver measure, Ma Mopar built 2 A body cars with fuel economy clearly in mind. They were the Dodge Dart Lite and the Plymouth Feather Duster. Both had lean tuned slant six 225 inch one barrel engines. Both had lightweight bumper brackets to save weight. Both came standard with aluminum cased 4speed manual overdrive transmissions and a 2.94 axle. They were actually EPA rated at 36 MPg highway!
Google it, you'll see that I'm not bullshitting.