I have a friend who works for Bosch (and used to work for Detroit Diesel and Daimler-Chrysler). According to some of the references he gave me (which I don't have handy), vehicles only create 8% of the CO2 emissions worldwide. It may be slightly higher, but the point is that auto-manufacturers are hardly the main culprit for CO2 emissions. You can look to factories (especially the pre-cold war factories in Eastern Europe and Russia which haven't been upgraded).
That being said, I still think investment in alternative energy sources for automobiles is a wise idea in the long run.
Funny thing about many self-proclaimed environmentalists - I find that most people are willing to talk the talk, but not walk the walk. How many of you drive a hybrid? How many of you recycle? How many of you use rechargable batteries? How many of you have signed up for your local energy company's alternative energy program?