Gil,

Your Engine and catalytic have to warm up before the system can optimize MPG. This happens in the frst few minutes of your trip. This causes low MPG at first which is then averaged out on longer trips. If your commute is short, you'll see a large hit to your MPG.

Another factor on very cold days is your use of heat to keep you warm. This cools the Internal combustion Engine (ICE) and limits the systems ability to run electric only which reduces MPG in low speed driving (there's plenty of heat generated at high speeds).

JeffD