You're blaming the wrong culprit. It's the winter gas' high Butane level. I think it's more noticable this year due to new polution regs.

Have you tried Lucas fuel treatment? Cleans the injectors and plenum and boosts mileage, noticably. My wife (she drives one, too) and I are averaging 330 miles per tank now, almost 400 in the summer without the Lucas then, to counteract the Butane which leaves us in the spring. Hope this helps.