The Union of Concerned Scientists (UCS) recently announced its rankings of the environmental performance of 2013 model-year light-duty vehicles sold in the U.S. by the top eight automakers. The 2014 overall winner, Hyundai-Kia, unseated last year's greenest manufacturer, Honda.

"Hyundai-Kia has made a concerted effort to improve the fuel efficiency of its fleet—and therefore reduce its global warming emissions—by turbocharging and downsizing engines utilized in a number of models. The company has also reduced its global warming emissions by introducing hybrid-electric versions of two of its top-selling vehicles, the Hyundai Sonata and Kia Optima," according to the UCS report.

According to the UCS, for the first time ever, all eight major automakers reduced their average global warming emissions compared to their respective 1998 fleet average, the model year examined in the UCS's inaugural report. The average new vehicle is emitting around 20 percent less global warming pollution and nearly 87 percent less smog-forming tailpipe emissions.

Click here for the full report.