fuel injector cleaners generally contain things like alcohols, napatha, things generally used in emissions passing chemicals, et cetera, that will raise fuel economy for the time that any significant portion remains in the tank. maybe it will go down 2 or so.
also...if you are about to need an oil change, those chemicals generally DO dilute your oil, so it may have knocked the hot grade of your oil down 10 or so, which would increase your fuel economy. this is providing that your oil was tired, though.
