My mileage is taking a big trip south!

Don't feel bad. You're not alone.
We can still enjoy our very good mileage but look forward to Spring when we'll again be getting over 50 mpg.

 

My mileage has also taken a hit in recent weeks. I'm down from 56 MPG to about 50 at the moment and it's still falling. :x

Causes include:
Winter tires all around
Winter formulation for gasoline
Cold weather
Higher friction in the cold
Lights on more
Snow and slush on roadways

 

It's gotten so bad for me I've put the Prius in the garage for the winter and gone back to the Hummer

 

Rob, you might be seeing the effect of 'winter formula' gasoline. I don't recall all the specs, but one of the kickers is there is less energy in a gallon of winter formula. That doesn't mean that the car needs less energy though. Actually, your current 47 MPG at around a horribly cold 50 degrees F is about what I was getting when it was warm here, about 50 degrees F. ;-)

 

As far as I know the petrol in the Netherlands and Germany doesn't have a special winter formula, so the hit in mileage isn't that big, but it *is* noticable.

I still think it's interesting that a difference in say 10 to 15 °C has such an impact on the performance of an engine.

 

My mileage has been doing the opposite in the winter now. I used to get about 40.0~42.0mpg. Now I am getting about 45.0mpg under the same driving conditions.

 

Hmmm, 6K+ miles, moderate temps. Yep, seems about right.