Milford, Mich.

SITTING behind the wheel of a 2011 Chevrolet Volt prototype on Wednesday, I found myself confronting what may be the greatest fear that future owners of electric vehicles will face: a battery-charge indicator showing just a few miles of remaining range.

If I were out on a desolate Interstate in a vehicle powered solely by batteries, I’d be praying to the god of electrons for a place to pull off and plug in a charging cord. But my drive is at General Motors’ proving grounds here, and I’m about to experience what the Volt’s vehicle line director (and my passenger), Tony Posawatz, says is the car’s trump card: a gasoline-powered generator under the hood.

Like other reporters, I had already driven Volt prototypes in the battery-powered mode, and they were predictably smooth and silent. But for eventual Volt owners, a crucial  and so far unanswered  question is how the car will perform when the battery’s charge is depleted and all electricity is provided by an onboard generator, driven by a gasoline engine, that has no mechanical connection to the wheels.

Will it be a slug? How annoying will the noise of the generator’s engine be in an otherwise mute car?