Ryan Avent ponders the greenness of car sharing:
Zipcar may mean that some trips which were previously taken on foot or by transit are now taken by automobile. But because using an automobile to ferry around huge items is so much more convenient than trying to do it on foot, the availability of car-sharing makes city life more attractive relative to the suburban alternative. And this should encourage more people to live in cities, which will indisputably be green, on net.
Phew. A zipcar is the only guzzler we use. And only when we have to move something big or heavy, or have to go somewhere outside of bike-range.