Hydrogen can be used to improve mileage. The hope with this method is that hydrogen will increase the amount of gasoline combusted and increase fuel efficiency, as you trickle hydrogen into the tank. Often propane is used to increase gas mileage. The studies are still being done. There's no official data at present, but there is significant experimentation.
Electrolysis is the method behind this theory. You'll need: empty soda bottle, electricity, graphite, jumper cables, and a salt solution made with baking soda. The electricity needs to be in the form of DC. Place one end of the cables in salt water and the other end connected to a battery. Attach the graphite to the end of the cable in the water.