All vehicles I've owned get lower mpg during the winter months. I see no reason why the Mirage would be any different? I think winter blend gas is a large part of it all. Other factors are surely in play even if you don't change tires during the winter months. Obviously, driving through snow or slush isn't going to help one's rolling resistance.
Early to mid-November is when I notice a change in my area, too. It may be a combination of different fuel & tires in my case.