What's the best car purchase to help the environment?

I am trying to work my way through the implications of this article from Slate, on the environmental merits of buying a Prius v. a used car.

In order to do an apples-to-apples comparison, let's pit the Prius against a car that's frequently cited as its closest nonhybrid equivalent in terms of weight, size, and other specs: the Toyota Corolla. Would it be more energy-efficient to buy a brand-new Prius or someone else's old Corolla? Since certified, pre-owned cars tend to be less than five years old and are refurbished before going on sale, let's generously assume that your used Corolla will last exactly as long as your new Prius: 11.5 years, or 172,500 miles. (The average American discards a car every eight years, but that's more often than necessary: A well-made vehicle will typically last 15 years.)

According to the federal government's 2008 fuel economy guide (PDF), a Prius averages 46.5 miles per gallon (assuming half of a driver's time is spent on city streets and half on the highway). Beyond 172,500 miles, then, the Prius will consume 3,710 gallons of gas. Each gallon contains approximately 124,000 BTUs of energy, so that translates into 460 million BTUs' worth of burned fuel. Add in the production energy, and the new Prius is responsible for a grand total of 573 million BTUs over its lifetime (not including disposal costs).

A Corolla with an automatic transmission, by contrast, averages 30.5 mpg—more than eight miles per gallon better than the average car on America's roads. Over the vehicle's lifetime, that translates into 5,656 gallons of gas containing more than 701 million BTUs of energy. Since the Corolla we're considering is used, we won't add to that total by factoring in production energy.

This ignores, of course, your impact on the market. As good environmentalists know, we are all part of the vast, interconnected web of the ecosystem. You cannot calculate your impact simply by estimating how much carbon you emit in your own commute.

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.

Me, I'm buying a used, little car and driving it very little, mostly because it saves gas and makes parking easier. But that's just the kind of selfish rat I am.