As more people become concerned about global warming and other environmental issues, many drivers consider the eco-footprint of their vehicle to be an important factor in making their decision. While miles per gallon is one way to rate how fuel efficient the car is, it doesn't get to the heart of the issue – namely, how much smog-forming emissions the car produces.

The Union of Concerned Scientists releases yearly rankings that look at all the vehicles that an automaker offers in a particular year, then names which one is the "greenest" overall. It does this by averaging various environmental factors and rolling it into a composite overall score. This year's study looked at cars from the 2008 model year, as it was the most recent year that full data was available to the group. The group only looks at the top eight companies in the U.S. – Honda, Toyota, Nissan, Ford, GM, Chrysler, Hyundai and Volkswagen – in determining the rankings, as these account for over 90 percent of the vehicles on American roads.

For the fifth consecutive year, Honda was named the greenest automaker by the study, but it just narrowly beat out competitors Toyota and Hyundai, who tied for second place. Just one point separated the three companies, with Honda scoring an 86 and the other two at 87 (lower scores are more desirable).

"It was a photo finish, but Honda is still the champ," said Jim Kliesch, author of the report.

Despite the victory for the 2008 year, Honda will likely drop off once the latest plug-in hybrids hit the market. While Honda has announced two plug-in models for 2012, those will likely come after many competitors have introduced their offerings. Honda believes the plug-in craze to be a stopover before the greenest technology is introduced: hydrogen. As such, the automaker is devoting more resources toward developing those cars.

"We feel that it's the ultimate solution," Honda spokesman Chris Naughton told CNN in July "But the infrastructure is developing more slowly than we had anticipated."

The study noted that many predicted Toyota to take the top spot from Honda this year, but the automaker stalled in adopting carbon-reducing technology. Hyundai, meanwhile, shot up the rankings by emphasizing more efficient vehicles with smaller four-cylinder engines.

The rankings were not good news for Detroit's "Big Three." Chrysler, GM and Ford finished in the bottom three spaces, with Chrysler placing dead last. The study did note that environmental standards were up overall and that the American companies could see their rankings climb thanks to future introductions like GM's Chevy Volt.

Drivers can likely find many of the 2008 models that were evaluated in the study on the used car market. Buying a used hybrid can make sense because the reduced price of the pre-owned market can offset the inflated price that many drivers pay when buying the vehicles new.