One thing I haven’t heard about is the use of acetone as a gasoline additive. Widely debated, it is asserted that a small amount of acetone added to a large amount of gasoline does not increase its octane level, but does break gasoline’s surface tension, which allows it to more fully evaporate and combust.
http://www.pureenergysystems.com/news/2005/03/17/6900069_Acetone/
I have been using it in both a 4-door sedan and an economy car for several years now, but only when the price of gas goes over $3/gal. I do notice considerable improvement to fuel economy in both vehicles.
Some will adamantly argue that it can’t work, it won’t work, and I will ruin my engine in the process. But from my own experience, it hasn’t happened.
Importantly, more is *not* better. Not a drop more than 3oz per 10 gallons, or you will start to lose mileage.
There’s all sorts of octane boosters out there. Many of them are aromatic hydrocarbons like toluene or xylene.
Bang:buck favors ethanol in many cases. Last I looked, toluene was running more than $3/gal when bought in 55 gal drums. Bulk ethanol costs less than that.