I suggest you take a look at a few things. Air filter ,fuel filter ,spark plugs and fuel quality your usng. I would leave the fuel filter for last as that is probably in the fuel tank along with the fuel pump.
I've tried premium (91 AKI) in the heat (110F and up). I don't say it makes no difference in mpg, but I can't pull a difference out of the normal tank-to-tank scatter. Scatter approaches 10% for me, despite very stable and consistent driving patterns, so one would not expect to be able to see a clear or provable difference in mpg. If it won't get me mpg, I don't quite see the point for my sort of driving. Maybe on a track? Maybe?