Prius fuel gauge is sometimes called a "guess gauge".

DO NOT run the car down to the last bar on the gas tank, or count on the little flashing "low fuel" light to warn you in time.

Toyota does not brag (or even officially admit) to the inaccuracy of their fuel gauge, but they DO say to refuel the car when the gas
gauge shows 1/4 tank remaining.

I would strongly suggest NEVER allowing the car to go below 2 bars remaining on the fuel gauge.

The good news is that there are some unconfirmed reports of Priuses (Prii?) being able to drive themselves up to two miles on the electric motor alone when the gas engine ran out of fuel.

NOT recommmended!!!!