Why did MPG drop 2015 toyota Avalon. Bad gas?

2015 Toyota Avalon 6 cylinder (not hybrid) 20k miles. Mileage shown on computer display dropped from 18.6 to about 15 recently...



When purchased new in 2015 city, hwy mileage was always about the same via the computer, 30 mpg on highway trips of decent length and 18.6 (give or take decimal point) city stop and go short runs.


Tire pressure always correct. Air filter changed yearly hardly dirty. Oil changed 3 times since purchase, which means it had new oil when purchased and then 3 changes since.


Tires new on front as of 2 year ago. Battery less than 1 year old. Air filter  60 days old.


So took a 14 hour trip one way and MPG per display was bout 30 mpg as expected. Same on way back.


With last fill up since trip the MPG in city / running errands drop from my expected 18.x to 15. I reset the computer and then changed to Sport and Eco mode and back to normal, which I read online to do. I always run in normal mode. Only had 1 fill up since trip and put on about 120 miles.


Perhaps bad gas?


Hate to throw money in fuel injection or something else if it is an anomaly....


Any thoughts?


Thank you very much

Peter