Best magnum MPG?

Cars weight and aerodynamics will be the ultimate limit to gas mileage and engines efficiency plays a big part but the engines we deal with have similar efficiency.

Which leave hp and engine size. I like to think of engine size dynamically instead of its static size 318/340/360 etc... dynamically an engine size is (rpm/2 ) × cid but the actually size don't matter so much so I just compare by multiplying rpm or gear size to cid and divide by the cid I want to compare to. Eg if you want a 360 to have similar mileage as a 3.23 geared 273 you'd just times 273 and 3.23 and divide by 360 which would equal 2.45. So a 360 would need 2.45 gears to get similar mileage to the 273 but would also have similar performance from the lost in torque from the higher gear ratio.