In 2009, Business Car claimed that Toyota was still the world's greenest automaker. That same year, the folks at Dow Jones named BMW the greenest automaker for the 5th time in a row. Apparently, an organization's methodology has a lot to do with automakers winning titles like this over and over, since the Union of Concerned Scientists (UCS) has just bestowed the Greenest Automaker Award on Honda, again for the 5th time in a row (the last time the award was given was in 2007).
At least this time, the race was close: Toyota and Hyundai (which lays claim to the most fuel-efficient automaker in the U.S.) tied for 2nd, and their rankings were just a single point behind Honda. The most polluting automaker? Chrysler, but both Ford and General Motors were contenders for the title – blame their heavy mix of body-on-frame trucks.
The UCS ranks automakers based on the scores of their "smog-forming and greenhouse gas emissions (primarily CO2) in its U.S. automobile fleet." The 2010 award was based on model year 2008 data, the most recent information that was available for analysis. You can read a summary of the report here (PDF) or just get the whole thing (PDF).