https://ecomodder.com/forum/showthre...nce-39987.html
I lost 7%, no changes to the car, oil cooler taped off and radiator partially taped off, and I noticed the bad mpg on a balmy 60F day. The difference is supposed to be around 2% before accounting for increased drag. Winter fuel runs better in a cold engine since it has higher vapor pressure, but the ECU doesn't care what fuel you put in when it runs rich on a cold start so there aren't really fuel savings.
I still can't explain the huge drop. I was using Costco fuel while the previous tank was 76, and Raley's fuel from Nevada seemed to be even worse. I still get Costco fuel when it's convenient because it's still a great deal even if something is "wrong" with it.