How far you can go before you need to refill your hydrogen reservoir depends on a few things; How efficient is the generator, how big is the reservoir and outside temperature etc.

Your question "How much water will it take a 33mpg car before HHO generator will go a 1000 miles after HHO gas, a 55mpg 1.9 liter engine?" If it is a HHO generator that does not have a reservoir I would check it every time you fill the tank. If it has lets say a 2 quart reservoir, I would check every other gas tank fill up.


Hope this helps. Thanks