If your in a cold winter state,then one of the main culprits is tire pressure. Cold weather sucks air out of your tires. So you might have one tire or tires that are lower psi then recommend. You can see it by taking an empty water bottle from inside your house and out into the cold. The bottle will start to squeeze bu itself as the air is drawn out.1 Hot summers,tires get hot,the air inside expands.

secondly,again if Your in a cold weather state,idling your car lowers gas mileage. If you start your car and let it warm up because it’s cold out,then your mileage will go down. I’m guilty of doing the same thing. No one likes driving in a cold car

Hope me this helps