You realize that lower octane gasoline has more BTU energy (power) per gram than higher octane, right? High octane fuel has Oxygenates added to slow down the flame front which reduces and resists engine knock (the higher the octane rating, the slower the fuel burns). Lower octane will give you better fuel mileage than higher octane will, all other things being equal, unless you have an engine knock, or a knock that you can't hear because the vehicle's engine management computer has already detected it and therefore is enriching the mixture and retarding the timing to compensate,
resulting in fractionally less power and efficiency. Octane is not a measurement of power, it is simply a measure of how effective a gasoline is at resisting engine knock. Using high octane will not give your engine a "treat". Over dosing on octane will only dent your pocket. Use whatever octane the manufacturer recommends.
If you are using the manufacturer recommended octane and still have a knock, moving up to a higher grade will reduce or eliminate the knock, at least temporarily, but all you are doing is treating the symptom, more than likely. If the knocking or pinging continues after one or two fill-ups, you may need a tune-up or some other repair. After that work is done, go back to the lowest octane grade at which your engine runs without knocking.
Also, don't think you can get anywhere near an accurate MPG reading off one or two tanks of gas, or worse,
this trip or
that trip of a few hundred miles or so. There are simply too many variables to make anything less than an average of 5 full tanks a valid indicator of fuel economy.
When you fully understand the hoops engineers jump through to wring every extra 1/100th of a mile-per-gallon out of the vehicles, you become less susceptible to believing in easy solutions to fuel economy, such as simply moving up an octane grade, or fuel saver magnets, or hydrogen generators, or platinum injection, or using that really snazzy 150-MPG carburetor invented by some guy now on the run from the automakers and oil companies. If it were that easy, manufacturers would be recommending higher octane, and everything else, in droves.
Read here: Car Talk | Premium versus Regular
And here: The Low-Down on High Octane Gasoline | Federal Trade Commission
