2 to 3 ounces of acetone per 10 gallons of gas gets an extra 2 mpg or more gas mileage for most people. It has something to do with better more complete burning of the gas. Works with diesels too.

Acetone is a comodity so it is not hype for a particular "wonder" product. It costs around $5 per quart or $15 per gallon.

Google it or go to

http://www.gasbuddy.com/Forum_MSG.aspx?master=1&category=1054&topic=153466&page_no=1

There are other gas saving topics there and they debunk rip offs like magnets on your gas line.