I do get Franks point, but he is also new to the site, you never know if they have ever been on a forum before or not, or are even aware of the search feature. Either way the OP knows there is a seach feature
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To answer the question, most fuel aditives do so little it isn't measureable. Only thing I can think of that does not apply are fuel injector cleaners. Fuel Stabilizers might work as well but that isn't for MPG.