Within the last year or so I read on an energy-related forum that a nip of Acetone added to the tank will increase gas mileage. Some may recognize Acetone by another name - old fashioned nail-polish-remover. I've tried this using Acetone from the hardware store NOT from the cosmetic isle at the local retail shop. It worked for me, to the tune of an (conservative) approximate 25 % increase in mileage in my 03 6 cylinder ford sedan as tested during the first two tanks each with approximately 1.5 ounces added (tank holds around 15 gallons). At the time I first tried this, my car was about 2 years old and already had about 25,000 miles on it.
As for "nip", the recommended dose is roughly 1 ounce per 10 gallons of gas.
This tip was posted a couple different times by a couple different people. I personally am not a motor head or otherwise all that inclined under the hood, so I cannot or have not been able to confirm the warning, but the warning says that use of Acetone may gradually damage or wear out the engine sooner than it normally would. My hunch suggests that car engines are not all the same, especially between the older tech - carburated, and newer tech - fuel injected, and between all the variables therein - 4 cylinder, 6 cylinder, 8 cylinder, american tech, european tech, asian tech, german tech, there may be a lot of variation in the truth to this warning. Then again, with gas at $3 a gallon and scheduled to go higher, for some people it may be worth the risk.....for others it may not be worth it.
If there is a gaoline combustion engine expert within reach of this post who can put the potential technical problems into simple but deatiled terms why this may OR will be harmful to the average engine, please do.