Gas gauge mileage

Maybe yes, maybe no. You haven't indicated driving conditions and whether the gas gauge has been recalibrated recently. You're really not going to know if something is amiss with the mpg until you've run a few tankloads through it. The fuel bladder in the Gen II in the USA leads to variations in calculated mpg from tank to tank.

I would suggest filling the car when it gets down to two bars left and calculating based on the odometer readings. After a few visits to the gas station you'll have a pretty good idea of the behavior of your particular car.

 

The North American 2004 to 2009 Prius has a bladder inside the metal gas tank to reduce emissions. It acts differently almost every tank, so single tank results are variable, keep a running average.

My rules on when to get gas:

If driving with the In-laws, get gas when it shows 3 pips. Do you really want to be stranded with them?
If driving with your spouse, get gas with 2 pips, so you do not hear about it the rest of your marriage.
If driving solo, and you feel like making 3 trips on foot to a gas station*, get gas at 1 pip.
If the last pip starts blinking, get gas NOW!

* it takes at least 3 gallons since the last time the computers were on to signal more gas, you can get gas 1 gallon at a time all day if you 'check' after each gallon.

 

it's the blatter. best thing to do is start tracking your tanks and fill up at 50% until you are comfortable with how much gas it is taking.