Not to re-hash old threads, but I came across this one and thought I should throw in my 2 cents.
I started using acetone in my '96 regal 3.8 last summer and kept tight logs on it. It is a very shaky science as I watched it go from about 30mpg to about 33 mpg avg. I got lucky twice with the right gas, from the right station, with the right mix all in one happy tank
. the two highest MPG's I was able to garner was one tank @ 35mpg, then once more a few tanks later @ 36mpg. Very hit or miss, and a fun experiment of which I have had no adverse affects. I have to say that there were almost NO improvements with winter gas (one to three mpg), which is another topic on it's own! How can you justify the ecological benefits of winter gas when you burn much more of it in the process?