Need help! Major mpg loss!

I have no experience with a full-size Prius, but I can tell from my 72K mile experience with C

1. I suspect they formulate gasoline for winter use in a different way, because even in a mild weather (as we had in VA a few weeks ago) I tend to consistently get worse mileage in winter, with all other things being equal. Some figures: winter time typically 50 to 55 mpg, summer 55 to 60.

2. Cold engine short hops are 40 to 45 mpg

 

In that case you have to take the car apart and put it back together again and see if any pieces are left. Lol. I am very glad you got that mystery solved.

Also the idea that one year vs another year would deliver different mileage is interesting but your recent results are encouraging. Having lost my 2015 I have been looking at older models.

I would bring it to a Toyota dealer just the same.

 

Yes, the cold weather, and running the engine to run your heater, hugely affect your milage. One strategy that helps is to drive until the engine is warm, then turn on the heat (as safety allows as far as window defrosting of course).