The gas is indeed different and that has an effect. But your car may need somewhat longer to get through its warmup sequence, and the whole time that's happening the engine is running in a very inefficient manner, so that has an effect. Cold oil is sluggish, cold tires don't flex easily, all these little factors add up and combine to pull your mileage down.

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It doesn't just appear to get lower mileage, it is getting lower mileage.

2 main reasons:

1) In winter months, stations use more Ethanol content. Ethanol is less efficient than gasoline, therefore reducing your mileage

2) Colder (denser) air draws more fuel then warmer air. Your vehicle maintains a proper fuel:air ration, so when colder air comes in more fuel is used.

Cold weather means that a richer gas air mix is needed on start up. The gasoline does not turn into gas so readily at low temperatures. So more is fed into the engine to compensate for this.

Running a lot of in car heating (or air con) increases fuel consumption - because some of the energy goes into that.

cold air is more dense. fuel map must increase to maintain stoichiometric optimum window. all engines use more gas when it's cold outside.

Gasoline is reformulated in winter months.

http://ask.cars.com/2009/02/whats-the-di...

I notice each year my high-mileage 2007 Sonata gets about 30 MPG in the warmer months and about 26 MPG in the winter for the same driving; it hasn't snowed yet where I live, and the tires are inflated.

Could it be that the gas is different, or some filter issue or just the gas is different in the winter?