Much of the pro and anti-hybrid "analysis" is either ideologically driven or designed to be contrarian to sell the story. You can always spin these facts. For example, it clearly would be worse for air quality and mileage to keep driving your gas guzzlers. But those cars are better than some cars left on the road that are even worse. Thus, the total effect of your getting a different car should also factor in the ability of your old car to be acquired by or transferred to someone who drives something much worse that should be taken off the road. Also, the used car market is great individually (although the price premiums have just about eliminated that given the repair and maintenance costs), but it's limited and can't satisfy demand. Buying better new cars also stimulates manufacturers to make better new cars, which increases the total number of cars on the road that are ecologically sound. Similarly, the price premium of a hybrid, for instance, has been overstated. Buying a Highlander and upgrading to a hybrid is probably wasteful. It costs a lot and the mileage advantage is quite small. On the other hand, very few Prius owners were in the market for a Civic or Corolla and decided to upgrade to a Prius. Instead, those are people who decided to ditch their Lexus or high-end SUV for a Prius. Thus, for them it is a clear cost-savings. They simply chose to buy smaller and cheaper.

You also have to account for the fact that old mileage estimates were inflated, sometimes grossly so. That 35 mpg Corolla might really be 30, which increase the mpg difference between it and a Prius. Also consider that from car to car, you might get cars that have been better or worse maintained, so your particular Corolla might not get 30, but 25.

That's just one example of why, as Anon alludes to, these substitution stories aren't nearly so simple as would seem from this post. The interplay between new car manufacturing, the new car market, repair and maintenance costs, gas prices, and pollution is far too complex for simple comparisons between BTUs to manufacture new cars vs. mpg for old cars.

It's true that you can't control the acts of others, so total effects are out of any individual's hands. On the other hand, it's not clear to me that used car markets can't satisfy demand. Isn't the whole point to change consumer tastes? I get this same pushback from my two-Prius friends. If you're willing to give in to the idea that people like buying new cars, then the case for the hybrid is easy. But if sustainability matters, then isn't that concession a little too easy? For instance, it may be plausible that manufacturing replacement parts for existing cars is a superior strategy.

My point is not that used cars are the answer, or that I know the answer. Jason is right that simple comparisons abound, but most do not include the possibility of not building more new cars. Comparing BTUs is too simple, but no more simple than comparing mpg b/t a new Prius and a new Hummer.

OTOH, I'm not shopping for a Geo just yet either. . . .

Fred, you're just upset because you know that I am better than you are because I'm a hybrid owner.

That's only one more reason you're better than me, Christine. So it's only marginally more upsetting. Just wait till I get my Geo. . .

I'm not sure that the used car market can satisfy demand. I searched for Geo Metros within 300 miles of Los Angeles on AutoTrader. I found five. Tercels? 31. (That's not just from the years listed, but all Tercels, and all Metros.)

For comparison: I found four Rolls Royce Silver Spurs in the same area and 85 used Hummer H1's.

I don't want to overstate my disagreement: I agree with Fred that people looking into cars have to take things into account besides MPG, because construction and shipping costs are important (and they're overlooked, just as they are in the food market). I just wonder whether it's as easy to find reliable, low-mileage, low-emissions cars (ideally non-death-traps) on the used market as we'd like it to be.

(Oh, just one more number: I found 1,588 used Chevy Suburbans for sale.)