The environmentally friendly vehicles of the 1990s were largely unsuccessful in their time, but with gas prices approaching the $4 a gallon mark, those econoboxes of yore could be finally coming into their own. Prices for used cars such as the Geo Metro and Ford Fiesta have risen by as much as 30 percent -- and more -- in recent months.

According to Kelley Blue Book, most of these tiny hatchbacks were selling for right around $1,000 just a few months ago, but are now going for as much $6,000 -- largely dues to high prices at the pump.

The Geo Metro is proving to be one of the most popular fuel-efficient vehicles of the '90s, thanks to its hybrid-surpassing fuel economy. The Metro was powered by a fuel-sipping 55 horsepower 1.0L three-cylinder engine, which returned 49 mpg. An even more fuel-efficient XFi model was also available, which dropped the Metro's power to 49 horsepower, but upped fuel economy to 58 mpg. In comparison, a $22,000 Toyota Prius can get about 45 mpg.

The Metro's wide variety of body styles -- including a three-door hatchback, a convertible and a four-door sedan -- is also lending to the car's recent success.

But what makes these vehicles so fuel-efficient should also be a cause for concern. Weight is the enemy of high EPA ratings, so cars such as the Geo Metro and Ford Fiesta did without several weight-adding safety features -- including ABS and airbags. "What is your life worth for that extra 10 miles per gallon?" asks Jon Linkov, managing editor for autos for Consumer Reports.