My 5.8 h.o. gets 9 mpg on highway. 4 or 5 in town. It's in a 78 f100 with c6 and 3.50 rear gears. I think it may be the holley sucking it all down but i need to something about it. The engine is from an 85 f150, if i knew it was going to be so bad on gas i wouldn't have bought it. Switched it over to a 2 barrel and it didn't help enough for the power loss.