We have something that looks like data, but is missing some key numbers.
First, you give us MPG and units of time. I.E week = 11 MPG and then 2 days ago, some unknown distance highway driving yielded 24.
Are you talking about an MPG reading on a display or actual calculated MPG?
The true test is fill up at a specific pump and stop when it clicks off. Then return to the very same pump and follow the same procedure. Do some math.
That is about as close as you can get to quasi scientific.
Did the free gas fully fill your tank, or did you have a certain amount of credit in fuel points and once you exhausted it, the pump switched off? Is it possible you didn't fully fill up with 89?
I'm not sure you have enough data to warrant concern at this point. If this was a recurrent theme with known full tanks of 89, then yes, there might be an issue. But if it's a one-off, you may have had enough circumstances conspire, such as a not completely full tank, colder weather, ice / longer warm ups, or more heavy traffic and idling vs the tank of 87.
Or, you could have just got a bad tank of gas?
I wouldn't lose sleep over it unless it was a recurrent problem.
Originally Posted by cwilliamsws6
A week ago when I went to fill up my car, as I took the nozzle out of the pump the hose smacked the 89 button which turned it on and I couldn't get the pump to switch to 87. I had enough fuel points that I was getting a free tank anyway so I went ahead and just got the 89. My fuel mileage for the following week was awful. My average was 11, and the highest MPG I got was 15 on a highway trip.
Now 2 days ago I filled up again, this time with 87, and almost immediately my city average went to ~16 and it got 24 on a highway trip. I'm not running a custom tune or anything crazy, so why did running 89 octane bring my MPG down so much?