I had a 74 Dart with a 225/auto. Got a best of 25mpg on the highway. However at least one piston had such bad blowby that the pcv air breather was gushing fumes INTO the air cleaner and clean oil would be dirty by the end of the week. But that thing just kept on going and did not too bad on fuel all things considered. Also I never did plugs, air filter, wires, or any of the maintinance items other than an oil change.