Originally Posted by
Cobrajet
As to your mileage, it will always go down in the winter. You are using more electrical equipment (heater, wipers, lights, etc), which puts an additional load on the engine. Plus, the cooler, denser air your car is drawing in will cause the car to run richer than it does in the summer. On top of all this is the fact that the composition of gasoline itself changes in the winter. We go from 'summer blend' to 'winter blend'. This is done to help with cold-starting and also to reduce emissions.