I haven't tried it in my VW but I'm continuing the experiment with my Saturn.
For the first tankfull it showed a 3 MPG increase. Since then I've only filled up three times. I have been collecting receipts but haven't done the math to get the exact MPG figures. I thought I'd have to have 6 or 8 tankfuls to get any meaningful answer, but based on miles per tankful I see a small increase (about 10 extra miles per tank).
A freind testing his mileage in his Saturn has showed an overall increase of 4 MPG. This is summer and winter combined and he feels it may reflect the larger diameter of snow tires over summer tires.
It's nice to think there would be a measurable difference but even if there is a small mileage increase, when you factor in the cost of the acetone the monetary savings may be minimal.
David H
'66 VW Beetle w/sunroof
http://tinyurl.com/qhw59"Where am I going ... and why am I in this handbasket?"