Fuel gauge remained full aft 140 miles or 1.78 gals

The extra click will contribute, as will individual pump shut-off variations.

But fuel also sloshes around in the tank, and the gauge filters out most but not all of this slosh. It can be tricked into dropping a notch prematurely, or into returning a notch that disappeared earlier. City stop-and-go, and fast winding mountainous roads, will create a lot more slosh than flat steady highway drives.

If you have a engine monitor (ScanGauge-II, Torq, and others), watch the unfiltered fuel level, FLV. It can move around multiple gallons very quickly, as fast as it can update.

The short answer is: Don't overthink this. The gauge is influenced by more factors than we can keep track of, causing the displayed value to have less value and meaning than you are trying to extract from it.

 

I'm used to getting ~ 150 miles from a full tank until the first bar of the fuel gauge drops, then bars drop off pretty quickly
The halfway mark though is pretty accurate.

If I care how much fuel remains in the tank without danger of running out, I assume an 11 gallon tank and use the tank MPG reading. This works for me since I always reset a trip meter and fill the tank when I refuel.

 

If you get a really good fill up you can get 150-160 miles or so on the first PiP (assuming 50+ MPG) but its downhill after that the PiPs go away every 20-30 miles

 

I have to reduce my assumed tank capacity to 10 gallons in cold weather in my 2004 Prius due to stiffening of the gas tank bladder (you don't have one in your 2012 Prius).

JeffD