http://en.wikipedia.org/wiki/Chevrolet_Volt#Drivetrain
This has been known since sometime around the initial release.
Some highlights of the conversation:
•Farah says that in his mind the Volt is unequivocally an electric car. "The Volt is an electric vehicle...because for the first 40 miles you can get full performance running on nothing but an electric motor until the battery is depleted," he said.
•The Volt has three distinct motive forces in it: a large electric motor, a small electric motor/generator, and a 1.4 liter engine. Up to two of those three forces can be combined in select ways through the Volt's secret sauce drive unit—given the road demands and state of charge of the battery—to drive the vehicle.
•Only the large electric motor is capable of moving the car forward on its own. The small electric motor/generator and the gas engine can only ever be combined with one of the other motive forces to drive the wheels.
•Even when the gas engine is on and partially driving the wheels, it cannot operate without electricity flowing to one of the other motors.
•The gas engine, under most conditions, will be used to drive the generator and produce electricity, and will not be used to drive the wheels.
•There is no "direct" mechanical linkage between the Volt's gas engine and the wheels, rather there is an indirect linkage that is accomplished by meshing the power output of the engine with the power output of one of the other two electric motors.
•Motor Trend's reporting that the magic cutoff speed of 70 mph is what the car uses to determine whether or not to make the engine to partially drive the wheels is incorrect. The engine is used to partially drive the wheels when the car calculates that it will be a more efficient use of the engine's power. There is no hard cutoff point.