Turned on my car yesterday and went from 7.6 L/100km to 9.1 L (~25.9 MPG), and then within about 5 minutes of driving I was at 15.9 L (14.8 MPG)!! Eventually after the car got warmed up it dropped to 9.6 L, and today sits around 9.1 L again.
Problem is I noticed my front driver tire looks like it lost a lot of air - almost flat. Last time I put air in them we were sitting at around 0 degrees C (about 32 F), and now its -20* C so I'm sure I lost a lot of density to the colder air.
Plus all the spinning/slipping while driving, and having to use my heater way more than I've ever used it... it all really adds up.
The last 3 days we've been below zero. I'm getting soft in my old age, so I've been using the remote start in the mornings (-15F), and in the afternoon (-5F). Other than that I've been driving the same and my in-town mileage is sitting at 20.8 right now. I'm making a drive down to Chicago tomorrow, so we'll see how it does on the highway.