73 Dart /6 MPG

I carefully documented mileage in my 69 Dart 225 from 1977-95, when gas was ~2x today's price, and I used fill-up to fill-up w/ gas pump reading and validated odometer to mileage markers. I got 22 mpg hwy and ~17 mpg in combined driving. Anytime you brake is wasted energy (ask a bicyclist) and driving short distances on a cold engine is inefficient. Driving 70 mph vs 55 uses ~30% more fuel in these cars, less so w/ the lemon-drop cars of the 1990's. Even my 65 Newport w/ 383 big-block got 18 mpg hwy. In both cars changing to Crane Cams XR700 electronic ignition didn't improve mileage. In the Newport, a Holley Pro-jection 2D didn't improve mileage over the 2 bbl Rochester carb, and was always finicky.

I think your 1973 has EGR. By 1974, they had more complicated vacuum controls of spark timing and such, at least in CA cars. Either of those can reduce mileage if not working perfectly. If yours has the air snorkle diverter, insure it is working. One friend noted their Escort getting much worse mileage and poor power on a trip in the 1980's and found the diverter stuck in the "suck hot air past exhaust position". Probably made the carb run rich. Moving it manually fixed that.