A. The U.S. Environmental Protection Agency has tested hundreds of these gizmos. Some have you add liquids or pills to the tank; others have you add an underhood magnet or a vortex generator in the air-intake system.
So far the EPA has found none that delivers significant savings; some can even damage your engine. The only devices that stand a chance of improving your miles per gallon are those that modify not the car but your driving habits.
Consider a real-time fuel-economy reader - such as the Kiwi ($299) - which installs easily on your dash and includes lessons to help you accelerate less aggressively and otherwise drive in a way that reduces your mileage.
NEXT: Dump your gas-guzzler...