Please disregard if you live in a temperate climate. 
After experiencing the past month of abysmal FE, I hereby declare this thread as a repository for those wintry blues, and a support group for those suffering from SFD (Seasonal FE Disorder).
Hi. My name is Rick, and I'm averaging 25.4 mpg this tank.
The first 100 miles of the tank looked great: about 30F and low 40's FE. Then it hit. +8F, cold start, 52 miles. 33 mpg isn't so bad, right?
Then it hits. 4-inches of snow. Not a lot, but enough to throw the entire city into mass hysteria!
Morning commute: Averaging 15 mph for 30 miles on the highway is good for 22 mpg. Yay.
The bad kind of turtle folks...
Evening commute: It looks good! The roads still stink, but traffic is moving nicely. Oh no! An ambulance... fire truck... Nooooooooooooo! Some #$%# in a Hummer spun out and created a movie-grade pile up. Well, nothing else to do with the next hour but listen to the radio, then pulse-n-slide. 23.7 MPG over 60-some miles today.
Tomorrow's another day, but I have to fill. Likely the worst tank in the last 3 years.
So, bring your tired drivers, your huddled mass transit dreamers. For life, Liberty, and the pursuit of FE...
RH77