“HHO generators” are the 21st century version of the miracle carburetor (suppressed by the oil companies!) that lets your car run on water. These claim to use electricity from the alternator and battery of a vehicle to electrolyse water into hydrogen and oxygen gas which, fed into the intake manifold of the engine, improves fuel economy by “up to 30%”. (Note that 0% fits the definition of “up to 30%”.)
For those unable to do thermodynamics or scam detection in their heads, Tech Ingredients sets up one of these devices and measures what it actually does.
To see how popular a scam this is, just type “HHO generator” into a Web search engine and follow some of the links. Here are two examples illustrating how slick this scam has become.