A while back when gas went into the high four dollar range there were some inventive folks who came up with a modification. They pre-heated the fuel so it became a gas before entering the carb. It increased their gas mileage. I don’t know how much but it seemed like a fifteen fold increase.
Nope. Gas engines are about 25% efficient, meaning they extract about a quarter of the energy generated by burning the fuel. So you could get, in theory, a fourfold increase in mileage. No more, though, and much less in reality.