As a chemist and car buff, adding acetone to gasoline is no better, and probably worse, than adding STP gas treatment.

Acetone at that level CANNOT reduce the surface tension of gasoline enough to allow for finer droplets to be formed in the piston chamber (one theory) and is not enough added oxygen (Acetone is 27% oxygen, ethanol is 34%) to have any increased burning efficiency (theory #2).

There is no common household chemical being hidden from the public to increase fuel consumption. Trust me, if there was the automotive enthusiast (especially racers) would already be using it successfully with enough data posted all over the internet to prove without doubt it works.

Best bet for fuel mileage is to keep your car properly tuned.