Originally Posted by gtferrari
the refineries are switching over to winter grade fuel. it is a cheaper fuel to produce. my mileage drops this time every year but not as much as you are claiming. maybe more around 3-5 mpg at the heart of winter. there are many more things to consider.
cooler air is denser. more resistance (drag) on the car. cooler air contracts so check your tire pressure. your car will waste more fuel at first start up trying to bring the engine and cats to operating temp (thank the epa)
Fords seem to be the worst at this ... they seem to race a lot more than any other brand on cold start.