Which of the big six auto manufacturers -- Daimler-Chrysler, Ford, GM, Honda, Nissan, Toyota -- has the overall best environmental performance? If you read the title of this post, you know the answer. For the last six model years for which there are full government records, Honda has had the best combined fleet performance in terms of both smog-causing and heat-trapping emissions, according to a new report (1.6MB PDF) from the Union of Concerned Scientists. GM, conversely, has the worst results, going from 4th place in 1997 to last place in 2003. Honda's smog-forming emissions in 2003 were less than half of the overall average, and their fleet greenhouse emissions were 82% of average; GM, by comparison, came in at 129% smog/104% greenhouse. Interestingly, Toyota, widely praised for its Prius, dropped from 2nd place to 3rd as their larger trucks grew more popular, with 84% smog/88% greenhouse, with Nissan moving to the #2 spot (70%/94%). Ford had the overall worst greenhouse emissions, at 107% of average, but moved to #4 on the basis of its smog emissions, only 91% of average.