Since nobody doing the auto installation is interested in doing the calculations beforehand, here are some data for you to use: 20.9 lbs of hydrogen is the energy equivalent of 55 lbs of gasoline. There are 50,957 Btu/lb of energy in hydrogen. Figure your gasoline consumption in lb/hr and you can figure your hydrogen requirements...at full hydrogen use OR at supplemental use.

It takes approximately 125 units of energy to manufacture 100 units of hydrogen energy. Therefore, unless you have some "free" source of energy (like the sun or geothermal), the net conversion is a loss.

Next research should be how much water it takes to generate a pound of hydrogen.