As indicated in earlier posts, the contributing factors are:
a) longer warm-up time (Not because you're letting it idle, but because it simply takes longer for the car to come up to operating temp.)
b) cooler operating temperature
c) fuel reformulated for winter

In theory, "b" shouldn't happen if your thermostat is functioning properly. But a lazy thermostat will cause slower corrections for temperature though. The reality is that it's cold outside, and your engine is going to run a few degrees cooler -- and that's going to cost you some efficiency.

BTW, the reduction in MPG has nothing to do with dense air and increased drag.

jp

'93 945 Turbo (201K miles), '93 945 (140K miles/I maintain for a family member), '93 945 Turbo (sold w 145K miles), '92 Mercedes 190E (174K miles), '85 Jeep CJ-7 w/ Fisher plow (225K miles), '53 Willys Overland w/ Fisher plow (sold to a loving home)