75/76 Dodge Dart /6 average MPG and improvements

MPG will be a balance of driving style, and vehicle configuration. Each physical item cascades on to the next one. 2.94 gear cars spin more RPM than they are comfortable with at 65, without the OD trans. The OD trans has the OD gear where #3 normally is, so the gear spacing isn't ideal (1,2,4,OD) and can lug a stock slant 6. There are taller gear sets (2.76 if I recall) but the expense is low end torque.

Best MPG on a near stock slant will come from a well TUNED slant 6. Carb dialed in properly. Quality ignition components (Consider HEI swap) Valve adjustment. Reduce parasitic loss (electric fan, synthetic fluids, modern tires at higher pressures, etc). Breathing upgrades such as air filter and low restriction exhaust. Add a vacuum gauge as a poor man MPG guide. Add all of the above, plus driving at a speed that keeps the engine RPM as low as possible without lugging the engine.

Best MPG on a modded slant is all the above, plus stuff like a shaved cylinder head to add compression, 2 barrel swap, cylinder head port work, headers, etc.