I have noticed the drop also.

In addtion to the cooler temps already noted, I would add that reformulated "winter fuel" is now hitting the fuel stations, this always drops FE in any vehicle. As I was commenting to someone the other day - a 15% drop on the typical 20MPG vehicle = 3MPG, a difference yes, but certainly not as noticeable as 15% decrease off of 53MPG, a whopping 8MPG decrease!

Also, in my case, my Tires dropped from 39psi COLD down to about 36psi COLD due to the lower ambient temps. As of yesterday, I am now at 42psi COLD and have noticed a slight improvement.

For me, other causes of the drop include STRONG headwinds, and increased rolling resistance because of rain over the last couple weeks.