I have a HHO generator(
smacks booster) we (some of my buddies) put it in my car to test it one night using a recommended electrolyte (lye) we powered it up and took a test drive. I would say the results were >2mpg. I was getting the same fuel mileage with 4 people as i normally would get with just me in it. Now I don't know how accurate this info is because the test drive was only 15min long. I took it out because I didn't have the wiring i needed (it was hardwired to the battery) I have reinstalled it and done all the wiring, but the lye ate at the sidewall of the pvc and put a coating on the plates so the generator is not working. I don't think this is the most efficient generator out there. I have heard there is a laser HHO generator which is very efficient (never seen it but want to). Also i have heard about super heating the water using the exhaust of a car to produce HHO (though plasma?) Some other thing are the GEET fuel processor and the PICC (I would say they are both using plasma) Stanley Meyers had a water powered car; I think he was using a mechanical way of splitting it (He die/was killed/was poisoned)
About the car manufactures having the technology. There are reports of the Aries K (Dodge) getting 80mpg. I have also heard of a guy who had a Ford Bronco (guessing a early/mid 80's before because it has a carburettor) getting 60mpg. The owner of the Bronco was proud of his vehicle's mpg and spread the word. When Ford found out they left one slip out of the test site they forced/replaced it with a newer model that got worse gas mileage. Now I just have to think how much better and more fuel efficient fuel injection. Another thing to look up is the Fish Carburettor.
guess I'll stop rambling