My experience with the fuel milage on small block versus big block in the same vehicle with the same transmission ratios and rear end ratios is with Ford 360 and 460 engines, but should be the same with a SBC and BBC. I got exactly the same milage when I drove them the same, but would get significantly fewer MPG when I used the power available with the BBF. Pulling a loaded six horse gooseneck over the Grapevine, I could go 70 MPH with throttle to spare, whereas the 360 could do about 40 full out. Trade off was 2-3 MPG with the BBF while doing so. If I had slowed to 40, I think it might have gotten better milage than the SBF.
