Lots of con artists are waiting to sell you gas saving devices that don't really work. The only way to really know what effect a device or fuel additive has had on your vehicle is to run it on a dynamo and measure it. The Mythbusters test out three supposed gas savers. One is a magnet, another is using acetone as a fuel additive and the third is a carburetor claimed to provide hundreds of miles per gallon. Each one is found to either have no effect or actually reduce gas mileage. Watch and see!