Trying to understand where my MPG went

Get a bottle of fuel-treatment and do this reset

 

Another potentially BAD assumption.

While a 12 V battery going bad usually will trigger a code and warning light, sometimes it won't but
will just result in a malfunction in a sub-system with no warning lights.

The 12 V system runs ALL of the computers and associated electronics.

 

yep, it's all about outside temps with ev's

 

maybe it's not warming up properly

 

I have a 2012 Prius 3 and it made a rattily start one day. Shook like crazy then ran ok (did this several times). I figured it was missing on start up when cold so I changed the plugs. The missing was cured but the mileage dropped from the 48 to 50 mpg range to around 44 to 46 range. I initially thought it was because the plugs I used were the new recommended ones from Toyota that were slightly hotter. I kept thinking about it and decided it was unlikely the plugs as Toyota engineers would have know that would happen and either notified us or not changed to a hotter range. So I changed all 4 coils thinking a spark could be weaker on one but not weak enough to trip a light. On the next tankful the mileage was back up to 50 mpg and has stayed that way since. It has been over 20k miles now.