318 pistons

needsaresto,
Your fuel mileage will be determined from the size of the port and valves used, smaller ports and valves the better the mileage will be, just as the larger the worse.
Compression will determine the amount of power and the amount of octane that will be needed and not how much fuel that will be burnt.

Example: I have a large port engine with a large port intake manifold, and it uses 1+ gal per run and I have a small port engine with a small manifold and the engine uses .5 gal per run and makes more Tq. and low end Hp. Both engines are 10.5 compression and similarly built but the amount of area that needs to be filled to correct the fuel curve was totally different. And had to change this or the engine would have run lean.