A vintage Vespa looks cool, but the scooter of your dreams may not be as green as you think. Although scooters are more fuel-efficient than cars, older gas-powered models--and even some new ones--pollute more than the car they take off the road.

Older Vespas and the cheapest new models (including Vespa, Yamaha, Suzukiand MZ) run on two-stroke motors, which combine the engine's four processes into two steps. The simple design has offered low-cost mobility for 70 years, but it's inefficient. Unburned fuel leaks out of the exhaust on every stroke. An EPA article says that when runoff pollution is considered, some two-stroke motors can pollute more in two hours than a car would in an entire year.

You can still scoot and be green on a four-stroke scooter. All of Honda's new bikes are four-stroke, including the affordable and hip Metropolitan. When buying used, make sure to ask the salesperson which type of engine your scooter has.

The greenest of all? Electric scooters. Most are slower than gas-powered models, but still great for around the neighborhood. And, coming soon: liquid petroleum. These (LP, propane, autogas) scooters are popular in Asia and run very cleanly with excellent performance, but they aren't yet readily available in the U.S.

Find electric scooters at The Green Car Co. and MC Electric Vehicles. If you just want to give scooting a try, rent a four-stroke model at Scoot About.