[aerohead]

Reinforcing comments already made,and adding to them, colder temps mean:
*engine coolant will never reach temps of summer operation even with the thermostat operating.Thermal efficiency suffers.
*engine oil never reaches an operating temp (viscosity) equal to summer driving.
*transmission fluid or gear oil never reaches temperatures (viscosity) of summer driving.
*differential oil never reaches summer temps (viscosity).
*wheel bearings,u-joints,C-V joints operate under higher viscosity.
*just for all the lubes to reach whatever equilibrium temperature they'll settle in at may require 30-miles of driving,rather than 22-miles in warmer weather.
*rubber belts are stiffer and offer higher drag.
*rubber tires are stiffer and offer greater rolling-resistance.
*air density is higher,increasing aerodynamic drag.(16% between 70-degrees F and 0-degrees F,and 7% mpg loss alone!)
*engines,when shut off,will lose more heat between restarts.
*you may be operating more accessories (heater blower) and lights (daylight visibility) which will load the alternator.
It's a real can of worms!