Greenest cars: Natural gas Honda Civic GX, Nissan Leaf electric and, just barely, Chevrolet Volt

For a list of the greenest 2011 model-year vehicles, there sure are a lot of traditional gasoline engines among the top 12 cars ranked by the American Council for an Energy-Efficient Economy.

The council's 14th annual ratings of the most eco-friendly cars on the market include internal combustion models such as the Smart Fortwo, the Ford Fiesta and the Hyundai Elantra.

The natural gas-powered Honda Civic GX topped the list for an eighth straight year, with a score of 54, followed by the new all-electric Nissan Leaf. Other vehicles considered to be the greenest of the crop included the hybrid Toyota Prius and the hybrid electric Chevrolet Volt, which squeaked into the last spot.

The electric version of the Smart Fortwo would have pulled in a score of 60, but with only 250 units of the 2011 model available in the U.S. (and then only on a trial basis), the car didn't meet sales requirements for the list.

"Vehicles running on electricity emit nothing from the tailpipe, but their 'upstream' emissions can be substantial, depending on where they're charged," said Therese Langer, the organization's director. "As U.S. power generation becomes cleaner, these vehicles' scores will rise."

The battery manufacturing and disposal processes - which can be emissions-laden and toxic - also were factored into the calculations and knocked electricity-dependent vehicles down a few rungs.

Diesel vehicles missed out entirely. Other talked-about cars, such as the electric Tesla Roadster, didn't make the cut.

The list of gas guzzlers - known as the "meanest" - included heavy trucks and sport utility vehicles instead of the rash of European sports cars that has dominated in the past. Still, the Bugatti Veyron, one of the most expensive cars in the world, led with a score of 19.