Megan McArdle shows at least four hands:
The supply of used cars is pretty well fixed–they have to be in pretty horrible condition before they’re junked rather than resold for a pittance. So the correct calculation is not how much you will emit by driving one, but how much you will emit compared to the person who would have bought the car.
But then, that person would probably have bought another car. If they would have bought a Prius, you’ve simply swapped places. If they would have bought another car, you’ve increased demand for a less fuel-efficient option.
On the third hand, as far as I know most industry analysts still believe that Toyota breaks even, or loses money, on the Prius, and so the normal price signal sent by buying a car–"increase supply of that model"–may not operate. If the person who would have bought a used Corolla instead buys a new Corolla–or someone far down the purchase chain does–you’ve probably done more for the environment than you would by buying a Prius, because you’ve actually increased the supply of fuel-efficient cars.
In fact, it seems to me that the best option is to buy a used SUV and drive it very little. But I have a feeling that this would not give a potential Prius owner everything they are looking for in a car.