For some reason, using a generator to power an electric motor is more efficient than an internal combustion engine. The volt should get better fuel economy when using the generator than a comparable internal combustion engine car.
Just look at RV motorhomes for some insight. For some reason, RVers get better fuel economy running their roof air conditioner off a generator than they do running the air conditioner that is installed in the engine compartment.
I guess the reason might have to do with specialization. Most internal combustion engines have to perform many tasks (pushing the car, charging the battery, turning the fan, and power for the brakes, steering, computer, spark plugs, etc.), while a generator only has to do one job (charging the batteries), and can be designed to do that one specific task efficiently.