Prius Envy | Mother Jones
So how are we nonengineers supposed to know when to junk our old car without guilt? By Päster's numbers, building an average midsize car in 2007 emitted 18,000 pounds of CO2—about the same as burning 900 gallons of gas. He won't commit to exact numbers, but he does have a rule of thumb: If your jalopy is moderately efficient (i.e., gets better than 25 mpg) and you don't drive it much, keeping it is better than buying a new car. "But if you have an old car with pretty lousy fuel economy," he says, "then you're better off getting a new car because the emissions from making a new car are really not that big compared to the emissions from using the car."