There is a well-developed suite of electricity-generating turbines available to choose from - for sailboats:

http://www.cetsolar.com/airmarinex.htm

http://www.saver.com/product/sailboat_windpower.html

There's even a discussion on "Which is Better - Wind Generator or Solar Panels?"

http://www.e-marine-inc.com/articles/art2.html

But I'm pretty sure none of the manufacturers seriously expect them to be used on cars.