Driver for mileage

Dart Lite, to be determined.
I am driving on the first full tank of gas right now. As equipped with the 3speed, it does not like anything over 55 mph.
Stock 1 bbl (no accelerator pump), and intake, rattling exhaust that Y off under driver seat, to duals with glasspacks (Yuk). 3 speed manual, and 7.25 rear end.