Fuel Economy Changes from fill up to fill up?

Not trying to beat a dead horse here and ask what your fuel economy is but just wanted to get some feedback on my experience...

I have logged about 5000 (yes not a huge sample size) miles between my two '18 Tacoma's, both double cab 4x4 V6 automatics. I drive pretty conservatively, use cruise control alot on my 60 mile round trip commute to work. In both trucks I run 85 octane, I'm in Colorado fwiw, and I monitor fuel economy pretty closely. Up until I filled up 2 days ago they both got 20.2-21.8 really consistently per the on board computer. It's always very consistent and you can't really see/catch it when it changes.

On Saturday I filled up as a grocery store gas station(presumably not the best gas you can buy?) and since then the computer A) Clicks up and down a few 10th as I'm looking at it...can be accelerating, decelerating, or idling. Before it seemed to be more calculated before making changes and B) I'm averaging 16.7 so far on this tank after about 200 miles.

Side note/question: Specifically at altitude can any Tacoma guys chime in and let me know if running 87 octane affects fuel economy? In my '17 Turbo V6 Ford F-150 you would gain 1-2 MPG's going from 85 to 87 and the same going from 87-91.

Bad gas? Thoughts?
Thanks.
Mark

 

Regarding fuel octane, you need enough to prevent spark knock or predetonation, but anything beyond that will generally hurt fuel economy. Lower octane fuel ignites easier, meaning it is more likely to completely burn. High octane gas may not completely burn in a lower compression motor before the exhaust valve opens.

However running too low of an octane fuel will cause detonation. The computer has sensors to detect this, and it will pull timing to help prevent engine damage. Pulling timing will kill fuel economy as well.