This is no Joke. The difference in winter is that it gets Cold!
So in the northern states, maybe the whole country, they get winter blend fuel.
It has to do with the amount of Butane in the fuel.

A 30 sec. google search found this ;
http://auto.howstuffworks.com/fuel-e...mmer-fuel1.htm

 

That's funny you ask the question. Then make fun of the answer.

 

A couple of other things can cause a drop in MPG's due to the cold temp.
1. Takes the engine, transmission, differential, longer to get to normal operating temp.
This means you get worse mpg's until they are up to temp.
2. A lot of people start there vehicle to let it warm up before they actually drive it. Even more so now with most vehicles having remote start.
So your letting the vehicle sit still and run for 5-10 minutes. Getting 0 MPG's
3. As temps go down the PSI in your tires will drop as well. If the temp davg goes from 90 to 50 degrees you could lose 5 psi.
Low PSI means more rolling resistance. So air up your tires when it gets cold.

 

gasoline is blended to be heaver or lighter as to the air temp expected in the next 2 weeks. with colder air the gas will be blended lighter, IE evaporate easier. check out "reid vapor pressure"