back in the early 70's, i had a 64 barracuda with a 273 2bbl and a pushbutton 904. it had an open 7 1/4 rear, but i don't remember the gear in it.
running back and forth to work on the interstate at 65-70 [this was during the 55 mph speed limit time] that thing got around 26mpg.
was a really sweet running car until my old man clipped a phone pole............