Obviously missing something but is there a range indication for the car based on MPG and fuel in the tank.
You'd be well advised not to rely on it too heavily as the number will change quite dramatically with road speed, hills and a few other things. My recommendation would be to fill up before you get down to 1/4 tank if you plan on a long journey, but there is a low fuel warning light comes on when there's about 1 gallon remaining. And welcome to Prius Chat from across the Tamar.