Predator part #...339351.....9r09....2006 mustang GT

My GT is stock except for an S&B cai. when i install a tune, 87,89 or 91 octane, i tried them all, the car runs good but my mileage goes down by 4 to 6 miles per gal. when i load the stock tune back into my car then the idle rpm fluctuates about ...