The Civic GX, which runs on compressed natural gas, gets the ACEEE's accolade of greenest of the green. Mileage figures of 30mpg (city) and 34mpg (highway) are propped up by the car's EPA emissions rating, which is the lowest of all vehicles on the road. As well as being certified as an advanced technology partial zero emissions vehicle (AT-PZEV), the GX also is rated by the EPA as an inherently low emissions vehicle (ILEV), meaning it conforms to standards for zero evaporative emissions.

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Toyota's iconic, egg-shaped hybrid is a familiar sight on the roads today. While the honeymoon period of long waiting lists and massive dealer markups may be ending, the Prius continues to be a hit thanks to its sophisticated parallel hybrid power system and excellent gas mileage. The Prius is rated as an advanced technology partial zero emissions vehicle (AT-PZEV), thanks to its ability to run on either its 500-volt battery pack or its 1.5-liter gasoline engine. If you drive carefully in the city, the EPA reckons you can squeeze 60mpg out of it.

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Honda's Civic Hybrid comes in at number three in the green charts. Unlike the Prius, which is able to run in electric-only mode, the Civic makes use of Honda's Integrated Motor Assist hybrid system, which works by having an electric motor integrated with the engine. The motor, which is powered by a battery pack, assists the gasoline engine to achieve improved fuel economy.

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Nissan is licensing Toyota's Hybrid Synergy Drive for its 2007 Altima Hybrid. Like the 2007 Toyota Camry, which will be its major competitor, the Altima Hybrid has a two-mode gasoline-electric drivetrain that enables it to run solely on battery power when engine demand is low.

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The Yaris eschews high-tech hybrid engine technology, relying instead on a dinky 1.5-liter four-cylinder power plant. Its engine uses Toyota's VVT-I intelligent variable-valve timing and electronic throttle control to minimize emissions and maximize fuel efficiency.

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Similar to its Yaris stablemate, the Corolla relies on a variable-valve-timed, ultra-efficient four-cylinder engine for its green credentials. Coming in at number six in the rankings, the Corolla gets an average highway mileage of 41mpg.

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The 2007 Toyota Camry Hybrid is a comfortable, well-appointed sedan, equipped with a bevy of technology, from the economical hybrid propulsion system to a raft of standard in-car devices, including a Bluetooth interface and a premium audio system. Like the Prius, it makes use of Toyota's Hybrid Synergy Drive series parallel hybrid system, enabling the car to operate on electricity, gasoline, or a mixture of both.

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Driven by a 109-horsepower, 1.5-liter, VTEC four-cylinder engine, the Fit is not going to win any drag races, but it's ideal as a commuter vehicle and for running errands around town. Despite its diminutive dimensions, the Fit is surprisingly roomy inside and comes with an MP3-friendly stereo.

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Kia's Rio5 compact wagon is motivated by a 1.6-liter four-cylinder engine that makes use of continuously variable valve timing (CVVT) to modify intake and outlet, depending on engine demand. For the most economical results, opt for the five-speed manual over the more primitive four-speed automatic transmission. The EPA estimates that the stick shift Rio5 will get an average of 32mpg in the city and 35mpg on the freeway.

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Hyundai gets two models in the top 12 cleanest cars of 2007, starting with the Accent hatchback, which is certified as an Ultra Low Emission Vehicle (ULEV). The EPA estimates that the 1.6-liter Accent will achieve a maximum fuel economy of 37 mpg on the highway.

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The Elantra goes one better than the Accent in terms of emissions, being rated as a partial zero-emissions vehicle (PZEV). Its bigger, two-liter engine means its gas mileage is not as good as the Accent's: the Elantra manages a fuel economy of 28mpg city and 36 highway--still remarkably economical for a midsized sedan.

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Even without a hybrid- or gas-powered engine, the Honda Civic makes it into the short list. Redesigned in 2006, the gasoline-powered Civic makes use of a 1.8-liter four-cylinder engine with Honda's next-generation iVTEC variable-valve timing technology. (The image here actually shows the 2006 Civic Si, which uses a more powerful two-liter engine: not as economical, but way more fun.)

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