Click and Clack had a column on that a month or two ago....ah here it is:
Dear Tom and Ray:
Last week my neighbor gave me an 11-page treatise on the benefits of putting a small amount of acetone in your gas tank: an increase of 25 percent to 35 percent in gas mileage with NO damage to the engine. The article tells how much acetone per number of gallons and gives the rationale for why it works. I am NOT knowledgeable in the workings of engines, nor do I have knowledge about fuels, so I can be snowed by people who may or may not know what they are talking about. I value your opinions. What do YOU think about this? -- Wil
TOM: It's bo-o-o-o-gus, Wil. With four "o's." Don't put acetone in your gas tank.
RAY: It does absolutely nothing to increase your gas mileage. We spoke to a fuel-systems engineer who works for one of the major oil companies. He said that because of all these rumors floating around on the Web, his company tested acetone in its own labs and found no increase in mileage. None. And he said the equipment is precise enough to detect anything over a 1 percent difference.
TOM: But it's worse than useless -- it's also harmful. Acetone is the primary ingredient in nail-polish remover. And while it will burn and is a high-octane material, it's also a very powerful solvent. So while it's in your fuel system, it'll be eagerly dissolving all of your rubber components ... like gaskets and O-rings.
RAY: In fact, some cars have a rubber hose in the fuel line that goes between the fuel-cap assembly and the tank. When you add your acetone, pouring 100 percent, undiluted rubber-eater right onto that tube, you'll be eating through it in no time.
TOM: Here's our final reason not to use it: It dissolves paint. So if you slip and spill a little bit outside your fuel filler door, you'll have a nice, unpainted line running down to the bottom edge of your rear quarter panel. When we see you drive by, we'll know you didn't take our advice!