One thing a lot of folks have been experimenting with is acetone in gasoline to do this. Of course there's patented and branded versions like oxytane (.com). The acetone is harsh and can swell most gaskets at least a little, but the ratio used is 3 ounces of acetone to 10 gallons of gasoline. That's 1:433. Reports of increased gas mileage by 20% are not uncommon, and smoother operation and slightly more power at top end. It makes sense to me, fuel is not completely vaporized before combustion takes place so some of it gets wasted or burned late in the power stroke.
Combustion efficiency is 98% or better in an engine these days. If it wasn't emissions would be far worse than they actually are. This means there is nowhere to gain 20% fuel economy simply from burning more of the fuel that is mixed in the cylinder. Universities have done studies with acetone and found no difference when acetone was added to gasoline.
http://www.kettering.edu/visitors/st...p?storynum=406
When I put Lucas fuel system treatment into my truck, 96 Dakota, it would average about 10% better fuel economy. Yes, I have over 100,000 miles of records to support it. The same additive in my 96 BMW 328 did nothing for economy.