I've had a few, none did that much better than a 318 in the same (A) body if there was much stop & go. As suggested, keeping the RPM down helps, as does driving it for mileage instead of like, say, a rental car...
The 170 we had in our '60 Valiant (stick) got much better MPG than the 225 I had in a '73 Dart (auto), which of course was much heavier and had more (Calif) anti-smog stuff. The Dart got about 18 MPG overall if I remember correctly, 50/50 mix highway and street. A '75 is almost certainly heavier due to bigger bumpers.