With two Dodge Rams, both 318's, one auto one 5-speed stick, I checked mileage for two tanks of gas each, using OD normally. Then I didn't use OD for a couple of tanks, driving as much the same as possible. Guess what? Not a bit of difference in mileage. Nada! None! Mostly country driving, 60-70 MPH cruising. OD reduces wear and tear and like that, but not much gas savings. Same thing with my '54 Bel Aire, with a '55 4.11 rear and OD three speed. It actually got worse mileage in OD, like 3-4 MPG less. But my '88 LTD 302 did get better in OD, so sometimes it works, I guess.