Here is a poll i did on the subject.
Don't expect too much.
http://67-72chevytrucks.com/vboard/s...d.php?t=172346
I've had a 3/4 ton longhorn, (4.10 gears) had a 300,000 mile old 350, never rebuilt, it got right at 10 MPG, then when I installed a high compression cammed out 350 in it's place, I got 13 average, and 16 MPG on 2 ocasions.
Next truck was a 69 SWB 1/2 ton, with 3.07 gears, had a well worn 400 small block, 9 MPG, installed same cammed out 350 from the first longhorn, and instantly got 10 MPG every day. Couldn't get any more out of it to save my life. (RPMs were too low)
3rd truck, one ton longhorn, 4.10 gears, caddy 500... 10 - 12 MPG.