The American Council for an Energy-Efficient Economy today released its 13th annual environmental ratings. As expected, hybrids and small cars dominate the 2010 list of greenest vehicles. The overall “greenest vehicle” title was once again given to the natural-gas-powered Honda Civic GX—although the CNG car continues to be a niche vehicle with limited availability. The 2010 Toyota Prius and the Honda Civic Hybrid claimed spots two and three.

The small 1.0-liter Smart ForTwo—the top-ranked car powered by a gasoline internal combustion engine—ranked number four, followed by the 2010 Ford Fusion Hybrid and 2010 Honda Insight. ACEEE assigns vehicles a single “green” score based on tailpipe emissions, fuel consumption, greenhouse-gas emissions, and a cradle-to-grave lifecycle analysis. Summary “Green Scores” of the 1,000+ configurations of all model year 2010 vehicles are made available to subscribers of ACEEE’s Green Book Online interactive database.

Diesels Fall Short

Clean diesels, including the Volkswagen Jetta and Jetta Sportwagen, barely missed ACEEE’s top-12 “Greenest” list. Clean diesels did not overtake hybrids in popularity, as some forecasters had thought. “While clean diesels once again perform well on our annual ranking, high prices both for the vehicles and for diesel fuel have kept them from really catching on thus far,” said ACEEE vehicle analyst Shruti Vaidyanathan. “They’re not having the impact in the US that they have had in Europe, and as a result, manufacturers are scaling back production and promotion of diesels.” Meanwhile, hybrids are expected to remain the fastest growing segment in the auto market.

The Last Year Until Plug-ins Arrive?

For much of the past decade, conventional hybrids such as the Toyota Prius have offered consumers the most compelling combination of environmental benefit, practicality, and drivability. The 2010 Ford Fusion Hybrid was last week named the Car of the Year at the Detroit auto show. Yet, the list of greenest cars could get shaken up next year as plug-in hybrids and pure electric cars come to market. Perhaps even the Honda Civic GX’s top ranking could be displaced by a plug-in car.

“With the Chevrolet Volt and Nissan Leaf both scheduled to make appearances in the latter half of 2010, manufacturer interest in advanced technologies is even more intense, with a particular emphasis on electric vehicles,” said Vaidyanathan. But at least for this year, “the eco-stars are once again hybrids and smaller, conventional vehicles.”