Thanks guys for the comments.

- I have been measuring my fuel consumption from September. so its not really winter thing
- For highway, I get almost 500 KM in a tank which is ~11 L/100 KM. Really good but not as good as my 2001 Grand AM.

I am okay with this mileage as long as i am keeping my Grand Prix. The only thing which i want to make sure is that if its the normal mileage or my grand prix has some issue