Electric motors and generators are pretty efficient, batteries slightly less. So if you want to use the gas engine as a generator:
90% generator efficiency
90% electric motor efficiency
80% battery storage efficiency
So 0.90.90.8 = 64.8% efficiency
You could improve the engine efficiency over normal use by keeping it in its powerband, but even so a 65% conversion efficiency will suck up the possible benefits. I would use the same method as hybrid cars, high torque electric motors for low speed and then gas kick in after 10 to 15 mph.