The 471 I ran in my 69 Charger would turn a consistent 16 mpg on the highway. No joke.

10.7:1 CR w/Edelbrock heads. 4.10 gears, 28" tires... and a Keisler 5 speed w/ 0.64 OD. Cruised at like 2800 rpm IIRC. Ran low 13's at about 110 granny shifting.

The key component was actually the 870 street avenger that I tuned to the edge using a wideband O2 and data logger (Innovate LM-1).

Got about 9 mpg around town. I hit 18 mpg once or twice on some long flat highway runs.