I use hydrogen (actually HHO which is a combination of hydrogen and oxygen). I get a 40% bump in my mileage with a big 5.7 liter Chevy engine, but with some of the new breakthroughs we're working on now, I should be able to double my mileage.

It's clean (burns emissions and particulates more thoroughly), it lowers temps of engine by about 5 degrees from our research, and uses about a liter of water a week to save 30 gallons of gas a month or more.

Works on everything but the Ford Powerstroke diesel.