Running a 59 Pontiac 389 which has been bored and stroked to 447 in my 59 gmc 1/2 ton longbed. Engine was a mild build. I have 3.70:1 gears and a 27% OD transmission. Gas mileage is still about 14 mpg at freeway speeds. I get about 10-12 MPG when I hotrod around and open up the other 2 deuces.
When I first put the OD transmission in, it was behind the 336 gmc engine. Mileage was not improved. I got 10-12 MPG with the factory 4 speed, and the same with the OD. The mileage did improve with the freshly rebuilt 447. I suspect my mileage woes with the 336 were due to a worn out Stromberg ww carb.
