Ditching your gas guzzler is a great way to reduce your carbon footprint, but if you really want to do something about global warming, get a used car. You’ll be putting less carbon dioxide into the atmosphere.

As Matt Power notes in this month’s issue of Wired, hybrids get great gas mileage but it takes 113 million BTUs of energy to make a Toyota Prius. Because there are about 113,000 BTUs of energy in a gallon of gasoline, the Prius has consumed the equivalent of 1,000 gallons of gasoline before it reaches the showroom. Think of it as a carbon debt — one you won’t pay off until the Prius has turned over 46,000 miles or so.

There’s an easy way to avoid that debt — buy a used car. The debt has already been paid. But not just any used car will do.

It has to be something fuel efficient. Like, say, a 1998 Toyota Tercel that gets 27 mpg city / 35 mpg highway miles. The Prius will have to go 100,000 miles to achieve the same carbon savings as the 10-year-old Tercel. Get behind the wheel of a 1994 Geo Metro XFi, which matches the Prius’ 46 mpg, and the Prius would never close the carbon gap, Power writes.

There are a lot of used cars out there that top 30 mpg, and more than a few that reach into the 40s. Just about any Ford Festiva or Aspire will deliver 33 mpg. The mid-’90s Mazda Proteges are good for 30 mpg or so, as are the Dodge Colts of the early ’90s.

Go back a little further and the Honda CRX HF models will give you 39 to 45 mpg in a sporty two-seater that’s fun to drive. If you don’t mind driving the cheapest of cheap econoboxes, the 1985 Chevrolet Chevette got 36 mpg and the ’87 Chevrolet Sprint got 40.

You may laugh, but these cars are becoming big sellers. USA Today says fuel-misers from the early 1990s are selling like crazy and prices have gone up 30 percent in the past few months. A rust-bucket Geo Metro with 175,000 on the odometer recently sold for three grand on eBay.

Of course, it can be tough finding a car that old that hasn’t been beaten like a drum, and Robyn Eckard of Kelley Blue Book tells us most used car buyers prefer something no more than 5 to 7 years old and with fewer than 100K on the odometer. No problem.

The auto industry has spent much of the past decade cranking out SUVs, but it has managed to offer a few highly efficient cars as well. The Toyota Echo delivers 30 mpg or better, as does the Honda Civic. The Chevrolet Prizm and Mini Cooper come close. Volkswagen’s diesels are good for mileage in the low 30s.

We’ve undoubtedly left some off the list. But the point is, you don’t need to buy a Prius — or any other hybrid, for that matter — to get great fuel economy and minimize your carbon footprint. You might feel better driving a hybrid, but you won’t necessarily be greener.