the net ouput at the drive shaft ( while my car is idling car if the AC compressor goes on I notice that the engine loses few rpm. On the road, I notice also that I need to press harder on the pedal to get the same speed (engine RPM). An alternator on a car can generate 50 amps at 12 volts (may be more but that's ~2hp at max capacity , since nothing is free is needs at least that amount of mechanical energy), what can this generate in term of electrolysis, how much fuel is consumed to generate that much energy.
the point is not to replenish the full tank, merely extending it. If burning the whole tanks yields a few grams (to move the vehicle few meters) it's not worth it but if it can produce say 1/4 of a tank that 1/4 max distance with a tank gained.
the electricity can come from different sources too, heat (muffler, engine block, which can help cool it -if it is a combustion- ) wind (small wind turbine, at highway speed ), braking (divert energy to shorten breaking distance, like engine braking with manual transmissions or some other contraption) ... etc.