Ok, I have an update. I filled up today, and here's my two pennies. I use the same station, same pump every time. I always run it till the pump quits, then I add a 1/4 gallon more. I figure that's the best way to consistently get an average. I do not exactly do city driving, but there's a lot of stop and go getting to and from work.
When I got the car I was getting just over 17mpg to and from work. I advanced the timing and run premium, and I was up to 18.5mpg or so. I then switched added the 3.73s and it went down a bit, cause I drive the crap out of it. After calming down a little and installing the Predator tune, I was consistently around 18-18.5mpg again. I added 3oz pure Acetone per ten gallons of 91 octane to the last two tanks of gas and I've gotten 19.25 and 19.17mpg. I tried not to change my driving habits at all for this test. That's an increase of roughly 1mpg. I did notice that I had been getting about 20 more miles out of a tank. I haven't tried this with highway driving yet.
I can't say that it's a huge improvment, but it saved me a gallon or so of gas. We'll go with 1 gallon saved per tank of gas. If I add 4oz of acetone to every tank of gas, I'll get 16 tanks from my 1/2 gallon of acetone. When premium is over $3 a gallon, say $3.15, and I only paid about $3.50 for this half gallon of acetone. I'll save an average of $47. That's the cost of a tank of gas right there. I'll run without it in the next tank, and see what happens.