I've had the same problem. Since the cold weather set in, and I got snow tires, plus my short commute, I've had a drop from 43mpg at temps of 50 degrees Fahrenheit down to 34mpg on my last tank, with outdoor temps in the 30's. I'm hoping my next tank comes out a bit better, but I figure over the course of the winter season, I'm only filling the tank about once a month, far less than I used to do with my old car, so I'm trying not to go bananas over it.

If you're not part of the solution, you're part of the precipitate.