The problem with some of these units is they might “work” as far fuel economy is concerned but also increase nitrogen oxide (NOx), a big no-no for a production, EPA certified vehicle.
A “private” person might not care...
Please do any tune ups of the engine prior to adding the hydrogen generators. Then run it for a while and get a good gas mileage figure for comparison.
One of the scams is to take a car, badly in need of a tune-up, install some worthless equipment and perform a tune-up, and claim the results as from the equipment.
Be extremely skeptical and cautious. I have a $200,000 engine dyno (I make airplane engines). I have tested Browns gas, it lowers the octane rating which retards the timing, not a good thing. If you accidently rev the engine you will destroy it. I have gotten good at destroying engines, lots of practice : )
What I have observed is that people who use these devices alter their driving behavior and tune up their vehicles. It is very easy to get better gas mileage simply by idling less, driving a little bit slower, not accelerating as quikly, etc. My dodge Cummins diesel pickup gets between 15 and 25 mpg depending on how I drive it. I think that driving into the wind as opposed to driving with the wind makes about a 2 mpg (10%) difference.
The bottom line is that gasoline is a very good fuel. Changing the composition by adding hydrogen and/or oxygen in any form has very marginal benefits, and a lot of potential bad effects. An engine is just a pump designed to run very close to the stoichiometric point, if you change the composition of the fuel too much be prepared to replace the engine.
I talk to these snake oil salesmen occasionally and I find the silence very telling when I tell them I would like to test their product on a dyno. More often than not, that is the end of the conversation, they lose interest in selling me the product instantaneously. I have even wondered if it isn't some kind of disproof of Einstein's theory of Relativity.
I would warn anyone against doing this to a vehicle you can not afford to replace. You are working with gasoline near a hot engine. Car fires are nasty ones.
I also understand that you can advance the valve timing (or retard the timing?) with the faster and more efficient burn of the hydrogen/fuel/air mixture. This usually causes higher emissions with gas and air alone, but you get better mileage- the reason Honda CRXs used to get 50 MPG and the best we can do today on average is about 35.
I could be all wrong, but this is my understanding.