i've tested acetone extensively on 2 cars. never have i gotten even close to 40%. that said, my gains of ~15% were made on one, but not the other. the only variable being the car achieving the increase is driven mostly hwy miles.
if you check the archives, you'll find that some here believe it works, while some do not.
This is something where, if you try it and it works on your car, great, keep using it. If it doesn't, then don't. It generally doesn't seem to help when various people have tested, but their are some cars or people whom it seems to work for. So, heh, if it works, keep it, if it doesn't stop.