Buying a fuel-efficient used car versus a new Prius

Update: I just found this article, which has the same point as mine, and assumes domestic production. It also pre-dates mine – that will teach me not to use google! As one who drives a 2000 Honda Civic, which in many real-world driving tests gets almost exactly 30 MPG consistently (I keep pump logs), I wonder if I would actually create a net reduction in gas consumption by buying a Prius or other car. Now, I’m assuming a new Prius, [...]