Welcome to Prius Chat. Hope you found your answer from Alan helped. Personally I have never bothered with that facet of the display. I'm just happy that my fuel bills are half of what they were with my previous cars.
[“So tell me HOW can the Prius make ANY kind of estimate about another car?”] • Because you tell your Prius this info [“Furthermore, HOW can the Prius accurately know "how full" I filled the gas tank?”] • Seriously?
Just fully charge the Prime and a full tank of gas. Use the electricity as the reserve for the gas tank. The EV range is very accurate so use it last. Bob Wilson
So I just found this thread. Whew. My 2020 L Eco just filled up first time. It asked for fuel cost then went away. So reading all posts here I have no clue what’s going on. But does putting in fuel cost only do anything? I like the info the car gives but have no understanding.
The instructions for what to do next are in your Owners Manual. I know it's a formidable task to pry open that huge tome and try to find what you are looking for. There is an easier way. The Toyota Information Service webpage has all the glove box manuals available for free download. They do charge for a download of the repair manuals. Try this page. The pdf version of the manuals is computer searchable.