Purchase 100% pure Acetone and add to full tank of gas or diesel at the ratio of 2oz. per 10gal. of fuel. Do not use cheap plastic container as a refill container as it will melt. I bought a Cutex nail polish remover bottle (4oz), ran the contents through and reused the container.
Results 1997 Ranger 172K 3.0L V6 @ 3.75 oz per fill up= Went from a measley 17mpg to 19.34mpg! Additionally the truck has smoother accelleration and spark knocking has all but vanished completely. As I understand it, the acetone acts as an agent to help disperse the gasoline in the combustion chamber increasing amount of fuel actually burned vs. sending unspent fuel into the atmosphere. Any simple search terms including "acetone as a fuel additive" will yield many hits, some with much comparo data broken down by manufacturer. *added benefit to diesel guys: reduction in smoke!