I had a fresh tune up in a stock 50 sedan with overdrive. Steady 50 mph in overdrive was 22 miles per gallon.
My 53 sedan with 215 OHV six and overdrive will get a little over 20 if I keep it under 65, but drops to 18.5 at 75.
Overdrive and slow speeds are your friend if mileage is a big concern, but how many miles do you have to drive to pay for the overdrive?