Friday Eco Question

Here's a question I've been wondering about:

If I upgrade my car to a brand new one that's more efficient, I save energy and emissions based on the miles I drive with the new vehicle. However, a lot of energy went into making and delivering the new vehicle, and, in theory, that energy wouldn't have been spent if I didn't buy a new car.

So how big of a MPG increase do I need to get before it's at least environmentally break even?