I think I have an idea that will surpass the HHO hype and it actually will contribute an improvement in mileage and reduce greenhouse gas effects.
Its called the TacoBell Methane Booster. You sit on a seat cushion that acts as an air filter. You then run a hose from the seat cushion to a place behind your airfilter. This will cause any flatulence to be directed to the intake of the engine where it will be burned as fuel and transformed into water and CO2 (note water and CO2 are less of a greenhouse gas threat than methane).
Once I get kits into production it will flood the market and end the hho scam.
Just imagine the gag gift potential. Its a polite way of saying to someone "You fart too much".