Just think about it.
Anything you add to gasoline that has fewer BTU's ( energy ) per pound than gasoline is going to dilute the gasoline and give you poorer gas mileage.

P.S. I think Dynamite, C4, and Plutonium have more energy per pound than gasoline... short of that, I don't know what else you could add. Adding Nitrous Oxide to your car adds more OXYGEN which allows more fuel to be burned per second, that's all. It is not, by itself, a fuel. And will make your MPG go down tremendously!

-John