I haven't noticed any increase in speed since the summer, but my MPG has fallen by 5-10% also. I have two theories:
1) As already mentioned, the fuel could be of a lesser quality in the Winter as opposed to the Summer, or atmospheric conditions may be less conducive to more efficient fuel burning.
2) (My belief) Now that gas is cheap again, I have less incentive to practice the same good fuel-efficient driving style I had several months ago. I drive a lot when I'm home, and not that much when I get to school in late August. When I got there gas was averaging $4/gal for me. Now it's about $1.75/gal. A combination of less overall driving (I didn't fill up once in the entire month of October!) with less incentive to drive smartly is why I feel my MPG is down.
I am also thinking that with the winter cold my car just does not get to operating tempature as it would in the summer months. It is using lots of energy just to warm itself up let alone me when we get going in the morning. My trips are short like 10 to 15 miles at a time or less. When it was in the Teens the other day on the highway it never got to full water temp. It actally stayed about 5 degs lower than normal on the highway.
What viscosity oil do you have in your car? If you're in that cold of a climate, it might be that your oil is too thick, especially before it warms up. This would cause your engine to use more gas just to move the pistons up and down, robbing you of power and reducing your fuel efficiency. Most motor oil will work at protecting the engine in both the summer and winter, but that doesn't mean it works equally well in both seasons.
You might want to check your owners manual for the factory recommendations in your current climate conditions. If you're currently running a 10W-40 or so, you might want to consider going to a 5W-30 or even lower. But as I said, check your owners manual to see what it recommends.