The weird thing about new or used, and co2, is that it is the total fleet that matters, rather than what we individually drive. Somewhere, at this moment, an old car is being scrapped. The worse it's mileage, the better.
If I say, sell my Subaru (what I had before the Prius), it doesn't get scrapped, it gets handed down the chain. Does it "push out" a clunker? Who knows.
(Of course, crashed cars are the biggest waste, esp. high mileage cars ... so drive carefully!)