Here's a video from Denver's CBS 4 about a "car pill" called PBI (BioPerformance Improver)
Yahoo!

This video will probably be active for no more than 2 weeks, so in case you miss it:
mpg without the pill: 17.67
mpg with the car pill: 17.65
Tests were performad at Colorado State University. BPI claimed an over 20% improvement in mpg.