Your engine uses about 1/3 of the energy found in gasoline to turn the wheels and the rest is dissipated in the form of heat and unburned fuel. The catalytic converter is an energy waster that burns up the unburned fuel to reduce emissions. It creates back pressure and further reduces fuel economy. If we can burn less gas more completely, then much better mileage can be the result. Water vapor can absorb the heat and burst into steam. Hydrogen can mix with the fuel / air mixture to burn more evenly and completely.
The only objective is to get more out of a gallon of gasoline

the battery must power the generator and it gets it's energy from the engine which is only 22% efficient at most. So it is best we get the most use from that 22% before it blows out the exhaust unless we use a fuel cell to recapture the waste energy. Regenerative systems always raise efficiency.
Ask yourself, why can't we build a car today that gets better fuel economy than a car built by Hitler in the late 1930's? 38 mpg was good for anyone and it was affordable. That's why they called it the Volkswagen. (The people's car)
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