Forbes reports that the cleanest cars on the road can be found by looking at the U.S. Environmental Protection Agency's scores for air pollution and greenhouse gas emissions (carbon dioxide, nitrous oxide and methane). For each category, the EPA rates vehicles on a scale of zero to 10, with 10 being best. The closer to 20 the combined score, the cleaner the car.

Topping the list of the cleanest cars available in America are the Toyota Prius and Honda Civic hybrid, both of which earn a perfect greenhouse gas score of 10 based on carbon dioxide emissions that impact global warming. They're also the only vehicles to earn air-pollution scores of 9.5 for tailpipe emissions.

Three other hybrids earned a combined score of 18.5 (9.5 for air pollution and 9 for greenhouse gas): the Toyota Camry hybrid, the Ford Escape hybrid and the Nissan Altima hybrid.

Domestically abundant natural gas is another alternative to petroleum-based fuel, and cheaper, as compressed natural gas cars pay about $1 less per gallon. The Honda Civic CNG joins the hybrids on the list with a pollution score of 9.5 and a greenhouse score of 9.

Automakers are hesitant about CNG cars due to lack of infrastructure in place to support it.

And there are even a few conventional, gas-powered cars have also performed well. The Ford Focus, Hyundai Elantra, Kia Spectra, Nissan Altima, Pontiac G5, Mazda3 and Chevrolet Cobalt all earned combined scores of 17.5. But, according to Forbes, they generally are not as fuel-efficient.