Something else I've recently found out by using a Scangauge is the majority of my FE loss in cold weather is due to being in "closed loop" versus "open loop" mode. I've found that when cold, the car takes longer to warm up the emissions components and stays in closed loop a lot longer. And even after the car is completely warmed up, the car stays in closed loop a lot more of the time than it does in open loop.

I don't recall the actual numbers but when I was driving at a constant 45 MPH with cruise control on a flat road during closed loop I was getting 32 MPG and when it finally switched to open loop I was getting 54 MPG, as displayed on the Scangauge.

Someone with experience with emissions control systems, O2 sensors, and the such should be able to explain why the car behaves like this. I'm at the extent of my knowledge.