For the fifth consecutive year, Honda’s natural gas-powered Civic GX took top honors as the greenest vehicle, according to the American Council for an Energy-Efficient Economy (ACEEE).

The Toyota Prius and Honda Civic Hybrid claim spots two and three, while the Nissan Altima Hybrid and Toyota Camry Hybrid also make the list of the ten greenest vehicles. “Hybrids stand out, even after being taken down a notch by the new fuel economy calculations,” said ACEEE vehicle analyst Shruti Vaidyanathan. The Smart Fortwo and the Toyota Yaris fall right behind the greenest hybrids, showcasing the environmental benefits of smaller passenger vehicles.

The ACEEE score incorporates unhealthy tailpipe emissions, fuel consumption, and global warming emissions to produce its annual environmental scorings of all model year 2008 cars and passenger trucks.

Diesels performed poorly due to the high levels of environmentally damaging oxides of nitrogen and particulate matter they release—despite offering greater fuel efficiency and therefore reduced greenhouse gas emissions. “We’re looking to diesels to help reduce global warming emissions, yet they’re still hanging out at environmental rock bottom,” said ACEEE Transportation Director Therese Langer.

The diesel-powered Volkswagen Touareg receives the dubious honor of being the year’s most
environment-unfriendly vehicle, leading a pack of diesel-powered vehicles that includes the Mercedes-Benz GL320 CDI, R320 CDI, and ML320 CDI, and the Jeep Grand Cherokee. Green car shoppers are eagerly awaiting diesel vehicles that can pass emissions standards in all 50 states—but the wait continues. Those vehicles with advanced diesel technology are not yet in the market, and therefore are still missing in this year’s ACEEE rankings.

ACEEE’s website, www.greenercars.org, identifies a selection of widely available greener models in each vehicle class.