The American Council for an Energy-Efficient Economy (ACEEE) has released its annual "greenest cars" list, and surprisingly, a hybrid or plug-in did not take the top spot.

It was the natural gas Honda Civic GX that ended up as the most efficient vehicle overall, beating out cars typically thought to be "green" like the Nissan Leaf and Toyota Prius. The reason? The ACEEE takes into account not only fuel efficiency and consumption, but emissions and the effects on the environment as a whole.

The group pointed out that hybrids and plug-ins, while emitting nothing from the tailpipe, cause a drain on energy when they're plugged in. They also lose points as their batteries cause emissions when made and will eventually end up as waste that's difficult to dispose of.

"As U.S. power generation becomes cleaner, these vehicles' scores will rise," said Therese Langer, ACEEE's transportation director.

The highly-touted Chevrolet Volt didn't make the top 10, coming in at twelfth on the list. Other top finishers included the Nissan Leaf in second, Smart Fortwo in third and Toyota Prius in fourth.

Drivers interested in a green vehicle may want to shop New Jersey State Auto Auction, as they offer a wide range of used vehicles, both hybrid and gas-powered.