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A generator might help if you had the hydrogen generator at home running off household current, compressed the hydrogen, and stored it in cylinders - if you're driving it off the alternator, you'll take at least 4 times as much power from the engine than you'll get back from burning the hydrogen. Also, it takes about 2.2 to 2.5 gallons of water to make enough hydrogen to replace 1 gallon of gas. If you take a look at most of the small hydrogen generators, they're way too tiny to get even a 10% improvement.