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)
2008 Vista Blue SES Coupe 5spd FRPP Zx3B Suspension
5000K D2S Retrofit
130,000+ miles and still on factory brakes and clutch!
Rep Points dont add themselves