'01 2wd 4cyl Auto... losing 25% mpg???

Since '03 purchase my '01 with 160k is right at 20mpg each fill-up, last two tanks are closer to 15mpg, what please do I look at for the source of this dramatic change?
Thank you!

 

Tire pressure.
Spark plugs.
Recent change in weather or quality of gas?

 

Spark plugs, fuel filter, air filter, stuck/dragging brakes

 

Winter gas kills my fuel economy in all my vehicles.

 

Thanks so much!
From your suggestions, I will confirm nasty snowy weather but that's not new after 16 years - so I will get the spark plugs and brakes (new rotors/front brakes in August) checked right away! (same plugs since 100k) - are there plugs that are suggested over others?

 

Also check the ebrake with the drums. They can get stuck partially dragging.