The Prius, which Toyota proudly describes as the first mass-produced gasoline/electric hybrid, is scheduled to arrive in U.S. showrooms in mid-2000. Its innovations represent an example of what continuing powerplant innovation will bring in the 21st century. "The simple goal with the Prius was to develop a car with much greater fuel efficiency and much lower exhaust emissions than any car in our lineup," says Toshihiro Oi, chief engineer for the Prius. "Our goal was to create a vehicle that would get consumers thinking about unconventional technology in very real and very practical terms." Using electronic controls, the Prius is able to run on either electricity or gasoline alone or a combination of both. The idea is to maintain the most efficient operating mode. Based on Japanese market testing and certification (more than 27,000 have been sold in Japan), fuel efficiency is double that of conventional gasoline-powered vehicles. Toyota claims 66 mpg and a range exceeding 850 miles with its 13.2-gallon fuel tank. Carbon-dioxide emissions are halved while carbon-monoxide, hydrocarbon, and nitrogen-oxide emissions are reduced by about 90%, Toyota says. The key to the system is a power-split transmission, which sends engine power either directly to the wheels or to the electric generator controlling the electric motor or battery state of charge. To maximize efficiency, the Prius is fitted with a regenerative braking system. When the vehicle is coasting or the brakes are applied, energy that normally would be lost recharges the batteries. Onboard computers integrate the powertrain for a seamless driving experience. Following Toyota into mass production of a hybrid vehicle is Honda Motor Co. Ltd.with its Insight, a two-seat coupe that is on sale in the U.S. this month. John Teresko, John Sheridan, Tim Stevens, Doug Bartholomew, Patricia Panchak, Tonya Vinas, Samuel Greengard, Kristin Ohlson, and Barbara Schmitz contributed to this article.