“ By using the gas engine to produce electricity, which then drives the electric motors, the gas engine can remain in its "sweet spot" - that is, the most efficient rpm for that engine… “ - Demosthenes
Then, why do we not see more automobiles with this propulsion arrangement with much smaller (lighter and cheaper) battery pack? Transmission (if any) and differential can be much simpler, too. Diesel locomotives have this driving system, by the way.
Just to be able to go 40 miles on batteries alone the Volt will be carrying a big, very expensive 16KWH battery pack. When the state of charge of batteries goes down to 30% its ICE comes on supplying electric power to the driving motor and to the battery pack, making the Volt a rather anaemic (53HP) heavy automobile. Suppose you start out your 200-mile trip with fully charged batteries, after the first 30 minutes or so you will be driving on the ICE power for almost three hours. I think GM should let us know of the fuel consumption rate in this condition. The Volt is a nice short-range commuter car, but certainly not a replacement for ordinary automobiles. Its ICE is for just in case. If your commuting distance is less than 40 miles, it is a great energy saver inasmuch as it utilizes otherwise wasted off-peak electricity.