The only way Acetone could improve your mileage is by cleaning your fuel system, including injectors or by adding it to your fuel and than tune your car.
+1 it reduces the surface tension of the gasoline making the molecules tend less to stick together. This allows for more efficient and complete combustion.
Acetone is not "corrosive" in the chemical sense of the word, but it's a very aggressive solvent for almost everything. In the world of chemistry, a shorthand rule of thumb is "like dissolves like". Acetone "dissolves" in water, and in gasoline, two very different substances, which means it has a wide range of solubilities, in both ionic and non-ionic substances.
"Plastic" is not plastic. There are many polymers which are called "plastic". The one most often used for gasoline cans (because gasoline won't dissolve it) is polypropylene (PP). In fact, acetone won't dissolve polypropylene either. PP is very resistant to solvents. However, not much of the "plastics" which are used in your fuel system are PP. As you'll notice, PP is quite hard and stiff, not suitable for uses which require a lot of flexibility and conformability. Nowadays, the uses which require more "rubbery" substances are filled by what are called "elastomers", which are usually co-polymers of two or more different small molecules, like propylene + ethylene + something else. These co-polymers do not always have the level of solvent resistance that PP homopolymer does.
Since the auto companies anticipated that you'd use gasoline in your car, they've spec'd the materials in the fuel line to be gasoline resistant. However, even now, most vehicles cannot use >10% ethanol in the gasoline, since the polymer parts in the fuel system can start to (1) swell out of shape from the alcohol, or (2) leach out additives from the polymers which will accumulate downstream. Acetone is a much more aggressive solvent than alcohol.
In short, you're nuts if you try to save a few pennies/gallon by adding acetone.
yea toluene is very easy to get actually. I get it from shirwin williams.lol But i only used it once my dad uses it now in the dirtbikes though cause it bumps the octane up to around 104. Pretty good stuff but its probably just as bad as buying race fuel that has lead in it.