"Continuing to drive an older car with poor fuel economy is less environmentally friendly than getting a new car that gets drastically better fuel economy. You can take my factors above and calculate the exact energy use for your old vehicle and a new vehicle to see the comparison. Keep in mind that these results are for the energy used, not the carbon dioxide emissions, but the two are highly correlated since most of our energy comes from fossil fuels."
Click over to Triple Pundit to check out the specific calculations, and to also read Pablo's take on the now infamous research that suggested Hummers were more sustainable than the Prius. Of course, one commenter also points out that your old car will most likely continue to be driven by someone else, claiming that this justifies holding on to the old car, and just driving it less. Having said that, the more people do this, the more the market will continue to ignore the need for cleaner, greener vehicles. Oh, it's a complicated world ::Triple Pundit::