Another reason that cold weather lowers mileage is that cold air is more dense (thicker) than warm air, so your car is literally having to shove aside a greater weight of air as it travels down the road when it is cold, which results in increased aerodynamic drag. If you drive the same route the year round and do a lot of freewheeling as you drive, this temperature effect on aerodynamic drag is very noticable and can result in as much as a 15mph reduction in your terminal velocity at the bottom of the hill when you crest the top at about 60mph between a 90F summer day versis a 0F winter day. There are many moderate downgrades on which I can maintain speed when freewheeling in the summer where I slowly drop speed when freewheeling in the winter. Also in the eastern US, the relative humidity (RH) is typically lower in the winter than the summer. Humid air is less dense than dry air, so the dry winter air takes a hit on your mileage.