Sudden mpg drop

[Tmchrstn]

I own a 2012 legacy 3.6r and I was averaging 20 mpgs give or take and when I went to get on the highway I noticed my car mpg dropped to 17 and thought no big deal but it only went down it’s currently at 13.5 I tried running fuel from a different gas station to see if maybe I just got some bad fuel but it did nothing 300 miles after a oil/ filter Change ... no check engine light no misfires no idea what it could be...anyone have any advice or ideas on it would be greatly appreciated!