At least until recently, when gasoline prices have gone up a lot, it has been more economical to continue to drive an older, less efficient car than to pay a lot of money for a new car. It turns out that it is also better from the standpoint of carbon dioxide emissions, as this article (that I got from "greenie watch")asserts:

CAR TAXES

An email from Dan McLuskey [dan_mcluskey@yahoo.com]

For what it is worth, it takes more energy to manufacture the average car than it will ever consume during its driving life. This means that the government push to drive motorists to buy new cars is the worst possible action with regard to emissions. The best action is to reward people for keeping their cars longer. Of course, if this is effective, it will have major ramifications for the viability of the motor vehicle manufacturing industry.