what's the most accurate way to determine MPG ?

That's it then. Everything you've shown indicates a problem with the consumption display or the the fuel indicator. Unless someone else sees something, I guess the only thing left is to fill up the tank.

... unless your car is parked on a decline on a steep hill. (I never run out of ideas.)

 

I think you're right.. the car was idled while at the dealer.. I remember we kind of discussed the dealer prep and how some dealers leave the car idling (or On) while they wash it, prep it, etc.

The car could've been left On/idling until it was a couple bars down.. then add another 20 dealer miles.. say, its down 3 bars now. Before the dealer delivers, they gas it back up since its now down 3 bars. Once fortytwok takes the car, its driven another 3 bars down.. so the HV/EV ratio screen shows about 6 gallons used, but its only down 3 bars on the gauge.

 

The way to confirm is a second data point with a reset of EV Ratio 1. With so little gas used. What is that? 10 more months to go? Wish my Prius had this problem.

Oh, by the way, in case you aren't aware, the owners manual recommends refueling at least 5.3 gallons of fuel every 6 months, page 178. So if its been 6 months and you've only used 3 gallons according to the gas gauge, you might have to expend some gasoline to comply.