Fuel Consumption Of 2007 Sentra

Hi Everyone.
I own a 2007 nissan sentra 2.0s CTV.
I got it in 2010 summer and the fuel consumption was acceptable which is arrund 9L~10L/100km(city) in summer time But it starts getting worse when the winter comes.

I live in Manitoba in Canada so the temperuture in winter time is like -20~-30 without wind chill.

I do know the fuel consumption would be worse in the winter time but my sentra's fuel consumption in winter time is like 12~12.5/100km which mean 50 liters of gas can only be using for 350km(215 miles). It is ridiculous...

So Im wondering if anyone owns a B15 sentra can share his/her fuel consumption in whatever season would be helpful for me.

Btw, I plan replacing the spark plugs to improve the fuel consumption . Anyone thinks is a good idea? Or anyone can recommend a good brand of spark plugs for 15B sentra.

Thank you guys for the help!!!