HHO is the gas resulting from electrolysis of water. Two H's and an O when seperated are very flamable. On top of being abundant it is cheap and cleaner burning than gasoline. Operating on purley HHO requires lots of modifications to a car but suplementing your gasoline with HHO requires little. There are a million sites out there on HHO car conversion kits. All of them are building plans for anywhere between 50 to 500 dollars. It seems odd that there is no instructable on it. So here, for free, is my attempt at making one. The text at the top is my first try and the text under the "-" line is my second try.
http://www.instructables.com/id/HHO-car-adaptation/