Are you in a hurry to get to work? If so, leave earlier in the morning. Speed kills mileage as well as waiting for stoplights in city driving. Yes, with a GT you should use Premium gas as that is what the engine was designed for. There must have been a reason that you bought a GT, unfortunately no one expected the gas price to rise that fast or stay as high as it has.
Speaking as a "Mature" person, I can remember when I drove across a state line (.05 mile)to save .02 a gallon. It was a whopping .31 in the state I lived it at the time.
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2008 Silver Steel Metallic, Touring, NA, Sirius; Chromed door handles; lower grille; fog light surrounds; chrome front & rear bumpers strips; Airaid air filter; Bassani duals; Modesty Cover; Autolite AP5263 Platinum Plugs (.040); Crane Plug Wires; NAPA Ceramic pads; Blane hood struts; PTeazer Gas, Brake & Foot Rest Pedals, '05 Rear Step Pad, Stainless Door Sills, Llumar tint, "A" Piller chrome; APC Strut Bar