Green cars: The un-dirty dozen

Four Honda models made the American Council for an Energy Efficient Economy's annual list of the most environmentally-friendly cars.

The Civic GX is especially green for two big reasons. First, it's very fuel efficient, getting about 36 miles per gallon-equivalent of fuel. The second reason is that runs on compressed natural gas, not gasoline. (Hence, the gallon-equivalent number.)

Natural gas is cleaner-burning than gasoline and it also contains less carbon than gasoline per unit of energy. That means less C02, blamed as a contributor to global warming, coming out its exhaust pipe. Altogether, that gives the Honda Civic GX the highest "Green Score" rank, a spot it has held for five consecutive years.

Since you're unlikely to find a natural gas pump at your corner gas - as in gasoline - station, Honda also sells an at-home pump that taps into an owners home natural gas line. Plan your trip carefully if you go far from home, though.

NEXT: Toyota Prius