[oldschool]

The ECM (computer) that runs the engine must adjust for the colder air, and add more fuel, too "richen" the mixture, this must be done or it would run like crap, in very cold weather that milage is not unusual. Also u are running havc and defrost hard that takes more fuel, also the alernator is running under a high load. Also do let the engine warm up for a few minutes? There is more fuel while not moving. Very cold weather kills fuel economy fact of life.