A "full hybrid" like the Toyota Prius saves gasoline in several ways. When the car stops--at a light or in traffic--the engine shuts off, and the batteries and electric motor run the vehicle's systems. When it is time to proceed, the motor propels the vehicle until efficient engine operation is possible, from 10 to 40 miles per hour. During hard acceleration or at higher speeds, the motor and engine operate together; a computer controller adjusts gear ratios between them so the transmission works at maximum efficiency, saving fuel. "The key is the controller, which makes sure all the pieces play nice together," says David Hermance, executive engineer for environmental engineering at the Toyota Technical Center in Los Angeles. In a "mild hybrid" or "hybrid assist," like the Honda Insight, a small motor helps the engine but cannot propel the car on its own. (Illustrations represent full hybrids.)

The downside is price. Hermance says the Toyota, Honda and Ford vehicles cost $2,500 to $3,500 more than comparable, nonhybrid models. This premium has limited sales to U.S. consumers, because few have shown they will pay up front for fuel savings later. But the equation is changing. The 2005 Prius averages 55 miles per gallon. For gasoline at two dollars a gallon, an owner will recover the added purchase expense in about four years and save money afterward--a more tempting deal.

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