The biggest contributing factor to MPG is what chris4x4 said, the cold warmup time, it uses a tonne of fuel
A few points to concider;
-tires & gear oils are stiff when cold= takes more energy to make them move= less mpg
-the AIR in the tires condences when cold=lower presures=harder to turn & effectivly a lower gear ratio also. Nitrogen is not effected by temp
-snow on the road takes more power,spinning on ice wastes fuel
-the cold air & fuel are actually a good thing, they are dencer & make the engine alot more efficient.
-You will still burn the same 14.7:1 ratio of air to fuel, so for any given amount of output power you are actually using a slightly lower volume of fuel. "very slightly"
-octane is like Rum, enough is great but more than enough isn't good.
-there are more BTU's in lower octane gasoline than in premium
(I live in northern Canada, in the winter I loose aprox 25% of my range per tank of fuel)