I have been using acetone in my Ranger on and off for around two years now and I can tell you it will not double the gas mileage. However, I do get an increase of 1/2 to 1 mpg (city mileage) when I use it. The strange thing is when I stop using it, the gas mileage does not drop right away. I have come to the conclusion that acetone cleans the fuel system and that is the major reason for improvement in mpg.
I disagree with the potential for damaging the fuel system and engine. The amount added is not enough to cause damage. I usually add about 2.5 oz/10 gallons. That is about 0.2% and is not concentrated enough to attack plastics or rubber. I am a Chemist and work with these kinds of solvents everyday. I do have a good understanding how they work.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.