[aerohead]

I just read your post,and input from Christ and I believe the timing may be the issue.All grades of fuel have the same energy content,and since the higher octane allows more ignition advance without detonation,and mpg is best at the highest timing advance,then the lower octane fuel may have triggered the vibration-sensor ( knock-sensor ) to retard the timing a bit to thwart detonation,and in so doing,move the engine into a less efficient timing regime.