I'm not sure I totally agree with the logic that keeping your old clunker is better than getting something like a prius because of the carbon footprint. This would be true if your old car was just dumped straight into a landfill. But you sell it used, and someone else drives it. If you hadn't sold it to them, they would have bought something else, and they'd still be driving, and I'm not sure that more used cars on the market creates more drivers. I guess you could argue that more used cars on the market brings the cost of used cars down, and makes them more affordable, but I don't think so, the prices are pretty well established. Even if this was true though, that somehow your used car on the market made it so that more people were driving, whatever small affect that has must be more than compensated for by the fact that by buying a high mpg car, you are showing manufacterers that there is demand for the mpg number.
just some random thoughts.
I drive a Subaru Legacy 99, it's green colored, beautiful, and gets 25-29 mpg. I can't drive efficiently though, I love moving fast... although I do try to take my foot off the pedal with red lights ahead, etc