Brandon - these guys have it about right, 20mpg would be really great with a 235 .... and I'll add this: if your 57 car originally had the 3 speed it would have 3.55:1 rear gears [with an OD it would have 4.11:1, we loved those back in the day for street racing] - if you're running an OD with the 3.55 gears you're likely bogging it down frequently on the highway, too low an RPM for good mileage, 15 would be about expected
Bill
