I drove to work one day last week in a prototype car that is either a harbinger of a far more fuel-efficient future, or another in a long line of technological insurgencies that will fail in the end to crack the auto industry's century-old status quo.

The car was a Toyota Prius modified, by the addition of a 72.5 kilogram (160 pound) lithium-ion battery pack, into a so-called plug-in hybrid capable of operating for as many as 40 miles almost entirely on electric power alone. The battery pack is a product of Massachusetts-based...