Where are these "numbers" you speak off. I've never heard of high octane giving better MPG. Premium is not better quality fuel.
"The fact is that the overwhelming majority of cars on the road today do not need a high octane gasoline to run properly and efficiently. Octane ratings are a measure of a gasoline's ability to resist engine knock. "Knocking" or "pinging" in an engine results from an uneven burning of the fuel-air mixture. If your car is not "knocking," using a higher octane fuel will do absolutely nothing for you but waste your money. The fact is that regular unleaded gasoline, 87 octane, is the recommended fuel for most vehicles. You can determine what fuel is recommended for your vehicle by checking your owner's manual.
If you are using gasoline with an octane rating greater than 87 and your engine is not "knocking," drop down to a lower octane gasoline. If your engine begins to "knock," go back up to the next octane level. Remember, octane is nothing more than a measure of anti-knock quality...use the lowest octane fuel which provides you with knock-free operation.
Another myth is that use of a high octane fuel will improve fuel economy. The fact is that fuel economy is determined by a number of variables, including the energy content of the fuel and the condition of your vehicle, neither one of which have anything to do with the octane level of the fuel. In fact, two fuels of identical octane could have different energy content due to a difference in composition.
One last myth is that the name of the grade of fuel is an indicator of the quality or octane level of that fuel. These names should not be relied upon when you are choosing which fuel to use. The fact is that you should always choose your gasoline by checking the octane rating on the yellow sticker on the gas pump. By so doing, you will be assured that you are buying the gasoline that you want and, by using the lowest octane that still provides you with knock-free operation, you can save a significant amount of money on your gasoline purchases."
http://www.co.suffolk.ny.us/webtemp3.cfm?dept=3&id=2049
