318 Magnum 70 Dart Gas mileage

It's not the engine but the gears from what I see. My LA 318 (but it is a 2 barrel not much smaller than the secondaries of a Edelbrock) gets almost 20 mpg. highway with a 3:23 gear and a 255-15 tire. Tall tire I know but.....what kind of mileage are you getting with those 4:10's? Even if it is a stocker, it probably burns tires for a city block! lol