[Ichneumon]

I'm just not buying this. How could that little bit of acetone increase mileage that much? It doesn't increase the fuel's specific energy (probably decreases it) any significant amount, and I can't see where it would likely make the fuel atomize any better. Nope, I'm not buying it.

One possibility: Maybe the acetone fakes out the O2 sensor into thinking the mixture is too rich, and it adjusts the fuel injectors to cut back a bit, causing the engine to burn a bit more lean?

[heckler]

The acetone reduces the fuels "surface tension" allowing it to vaporize more completely, thus burning more efficiently.
More complete burn of the available energy.