Non-linear.
For comparison, my '99 Hyundai Accent (average 35mpg, 10 gallons of usable tank capacity,) acts as follows:
100 miles: needle still beyond the 'F'.
150 miles: needle just at the 'low' end of the full pip.
200 miles: needle at about the 3/4 pip.
250 miles: needle at about 1/3.
300 miles: needle just above the 'E' pip.
350 miles: needle just below the 'E' pip, warning light comes on.
So when I'm halfway through the tank, it still says I'm almost full. In my Prius, it's nonlinear for the first 100 miles, (usually only 1 bar gone, or if I'm having a good tank, it might still say full,) then pretty linear for the rest of the tank.
On that note, today I had the first fillup where both MFD and gas pumped agreed that I got over 50 MPG over the course of the tank. (I've had it where one of the two said I exceeded 50, but not both, before.) The MFD dropped to 49.9 within sight of the gas station, so I EV-mode'd it around the adjacent McDonalds to bump it back up to 50.0. Gas pumped matched MFD calculation spot-on. The gas station attendant filled it one-gas-handle-notch short of full speed, and didn't top off (I'm in Oregon where there is no self serve,) so I'm guessing he either owns a Prius, or fills them up a lot.
