What was the fuel mileage before the overdrive?
Timing, vacuum advance, correct metering in carb, and fuel type all play a big part in fuel mileage. High ethanol content will kill mileage. Is the carb the correct one for this engine? or is it a general replacement?
Remember, you can not compare your car to a new car, rpm is not the only factor. Your '57 is a brick compared to the worn soap bar shape of new cars and trucks, that alone is most of the fuel mileage. 55 mph will increase your mileage greatly.
Joe
