Environmental guru Amory Lovins of the Rocky Mountain Institute has been preaching the virtues of lightweight automobiles (he calls them “hypercars”) for decades, with very little buy-in from the now-imperiled domestic industry.

As Paul Hawken described it in Mother Jones back in 1997, the hypercar would have “a superlight carbon-fiber body (safer than steel because it absorbs crash energy better), a scooter-sized engine, a gas turbine or fuel cell providing a constant source of electricity, and variable-speed reversible electric motors that can recapture braking energy for reuse after temporary storage in a battery or super flywheel. Quiet, safe, nearly 95 percent less polluting than a conventional car (engines running at a constant speed reduce emissions by 90 percent and such a light, low-drag car needs roughly one-tenth as large an engine as a regular car), the hypercar gets between 100 and 200 miles per gallon.”

It’s safe to say that Detroit did not build hypercars. Instead, it’s just brought out the new Cadillac Escalade Hybrid, so much the wrong way to go as 2009 looms. The company touts its 25 percent improvement in mileage, but that only gets the lumbering beast to 20 mpg in the city and 21 on the highway. It’s a hybrid, but it sure isn’t green.

Here’s a much better idea: the Toyota 1/x (pronounced “1/Xth”) concept. The thing weighs, get this, 926 pounds. With a plug-in hybrid drivetrain (as appears in the prototype appearing at auto shows) it can travel 600 miles on four gallons of fuel. With the same interior dimensions as the Prius, it weighs a third as much and has the potential of doubling that car’s already impressive fuel economy.

Lovins has long championed carbon fiber because its very light and stronger than steel. The 1/x has a carbon fiber-reinforced plastic frame, and a transparent roof made of bioplastics (kenaf and ramie plants are the source material).

The plug-in drivetrain, a quarter of the weight of Prius’ package, combines a lithium-ion battery pack with a 500-cc flexible fuel engine. Even the knitted polyester fiber seats are ultra low weight.

The 1/x is not brand new—it was shown at the Chicago Motor Show back in February—but it’s very up-to-date for what we need right now. As a concept car, the 1/x just has to look pretty up on the show stand. The hard part would be getting a production version to pass crash tests and meet other safety standards. But it shows that Toyota is thinking the right thoughts.

Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.

Toyota's lightweight approach to plug-in hybrids
For a really efficient plug-in hybrid car, let's consider carbon fiber and a drastic weight-reduction plan.