how many new tanks of summer gas have you run? might take a few to adjust.
i would try a different gas company as well, you dont have that much of a drop. but i know a few brands around here i will lose 3-4 mpg with the different additives, amount of ethanol.
any running issues? you said you gave it a tune up, that included air filter, plugs wires, oil? what else did you do?