Essentially every single sentence you just wrote, is as wrong as it is physically possible to be. "Burning" is an exothermic reaction in which a fuel combines with an oxidizer to release energy. In this case, the fuel would be Hydrogen. The oxidizer, would be Oxygen. Literally. The only thing that is happening, when hydrogen burns, is oxygen combing with the hydrogen to form water. That's it. That's literally the entire chemical process of what "burning hydrogen" even *means*. >isn't the effeciency of energy input equalling the energy output, but that water is a FREE resource If energy input doesn't equal energy output, then burning hydrogen is just a cost, that takes energy away from movement. It's just a pointless extra step that lowers MPG.