Acetone

[Super Fuel FX]

Upping the ratio above 3oz/10gal will actually cause your gas mileage to decrease. Don't know the specifics, but tried it and found it to be true. Also, as a word of caution, it is not recommended to use this in diesel fuel as it absconds (sp) water and takes it past the water collector and right into the engine, where water can cause so much more damage there than in a gas engine. Water doesn't compress like diesel fuel does.

[Realtyroy]

I run acetone and xylene and full synthetic oil in mine and I see about 3mpg increase. That's 3ozs of acetone and 3ozs of xylene and 2ozs of full synthetic 5w-30 motor oil per every 10 gallons.
It did take a couple of tanks before I noticed the increase in mpg's.