I call bullshit on any slant six automatic getting over 20mpg, especially in a 67 and up a body either you have a bad gas gauge, or speedos off. Best mileage would be with early a bodies with an overdrive no power anything, probably at best 22mpg. Maybe better with a 198 or 170. For me the biggest gain in fuel mileage was converting over to a 4 speed with a 225 went from 17mpg to 20mpg. I dont count my mileage from my speedo, or gauge. I carry two 1 gallon tanks of gas drive until i run out and count my mileage on gps. I bet if you guys did this you will actually see what mileage you get not just guessing and rounding.
