There is no panecea for improving your fuel mileage. This one sounds like urban myth...if it sounds too good to be true, you can bet it is!

You can try an additive-type fuel injector cleaner (the ones that cost a few more bucks tend to have a higher percentage of the detergent solvents)...or go and have your fuel system professionally cleaned if you are concerned about performance/efficiancy loss.

Cheers!