WASHINGTON, D.C. --- The American Council for an Energy-Efficient Economy has released environmental ratings for model year 2009 vehicles.

This is the 12th year ACEEE has published its rankings as part of ACEEE's Green Book Online, the organization's environmental guide to cars and trucks, available at www.greenercars.org.

This year's greenest title goes once again to Honda's natural gas-powered Civic GX. The Toyota Prius and Honda Civic Hybrid claim spots two and three, while the Smart Fortwo Convertible, the Toyota Yaris, and the Nissan Altima Hybrid complete the top six. Others on the Greenest Vehicles list include conventional and hybrid-electric vehicles from both domestic and foreign automakers. This year, the Chevrolet Cobalt/Pontiac G5 twins and Chevrolet Aveo land in the eighth and 11th spots, respectively, showcasing the improving performance of domestic vehicles in ACEEE's annual ranking.

Just missing out on a top-12 spot are the diesel-powered Volkswagen Jetta and Jetta Sportwagen, part of a new crop of light-duty diesels this year. These notable newcomers mark the official arrival of "clean diesels" that meet emissions standards in 50 states, previously unachieved by diesel-powered vehicles.

"We're finally seeing diesels in the running among environmentally preferable vehicles, thanks to new emissions control technologies. Now diesels' intrinsic fuel efficiency advantages can shine," said ACEEE vehicle analyst Shruti Vaidyanathan.