First timer, 08, possible gas gauge issues... hopefully nothing bigger

Well if your calculating really bad gas mileage I’m not surprised. Usually really low gas mileage at those miles is because of a failing hybrid battery. The expensive one in the trunk. Or the engine is shot.
Or little of both.

Failing hybrid battery at your vintage Prius is now the #1 post on this site. Watch the battery consumption meter on the mfd, See how quickly the hybrid battery level goes into purple bars which is really low.
If it goes into purple bars in a fast food drive through it’s time to either replace it or buy a discharge /charger system.

If I was you I would go to Toyota.com/owners and join that forum. Have your vin handy.
You will see every time the car has seen an authorized Toyota repair center at a dealer.
Lots of people have seen why there car was sold.

 

congrats and welcome!
gen 2 (2004-2009) have a rubber bladder inside the gas tank, which can sometimes be finicky during fill ups. or it could be a bad gauge/sender unit, or perhaps it just needs recalibration.
all the best!

 

Rather than leap to conclusions not supported by the facts in evidence, I offer this.

You may well have a gas gauge problem, if your reporting is accurate. The gas gauge will only register a change with a minimum add of 3 gals (in one go). You can force it to register the correct amount by disconnecting the 12 V battery for 30 secs then reconnecting. You will loose your radio station pre-sets, trip A/B readings, your accumulated average consumption and miles (on the consumption screen), your fuel trim values (although this is not a bad thing for a change of driver) and your auto-up/down setting on the driver's side window*. If you don't want to do this, the gauge should register the change itself eventually just driving.

If you feel the gauge is not working correctly, you could try recalibrating the gas gauge, if you have a nice level space on which it do it. Search here in the Gen 2 forums to find the procedure.

Are you aware of the gas tank bladder (only fitted to North American Market Prii) and the associated problems with it? Again, a search here of the Gen 2 forums will give you plenty of reading.

PS, I'd go with the on-board consumption meter for now, although it is not 100% accurate, I'd believe its 44 MPG vs your calculated. Mind you, if the gauge has not been manually reset using the reset button on the consumption screen, then this will be the car's average since it was last reset.


* To reset the auto-up, roll the window to half-way open. Then pull the window switch all the way up and hold it there. Once the window is in the full up position, continue to hold the switch for 3 seconds (count 1 international, 2 international, 3 international, in your head so you don't rush it). Release the up switch and now the auto-up/down should work.

 

These cars have a flexible fuel bladder and it can be slow to expand when the tank is close to empty. This is more of a problem in cold weather. This will prevent you from filling it as much as possible. What I do is open the fuel cap before I deal with the pump's payment system. This gives the tank a chance to expand while I fiddle with the card. This usually allows me to pump 7-8 and as much as 10 gallons at a time. Also beware that the fuel gauge is not super-accurate. Especially at the low end. It is very conservative and prompts you to "Add fuel" (with last pip of the meter blinking) way early. I usually put about 7-8 gallons max when gassing up soon after the blinking begins. Given that the car has an 11+ gallon tank I'd say it's a conservative warning. Last time I gassed up I decided to push it a little further and assumed 40 MPG (which is reasonable for me these days) and ignored the blinking pip until I was close to 400 miles on that tankful. I ended up pumping about 10 gallons of gas in. I was driving on the highway the entire tank and ended up with about 38+ MPG that time. I was not driving for fuel economy, obviously.

The best way to measure real economy is to run an average of your fill-ups and miles driven. Many of us here use Fuelly to keep track of the running average. You can see mine in my signature. It may be a bit behind the current fill ups due to the delay of the signature update, but it's close and represents my average MPG since I first bought the car last July. The MFD in my car usually reads a bit optimistic compared to what I measure at the pump, but not as far as what you are reporting. Anything under 35 MPG should be reason to be concerned. But first you must be sure your 18-20 MPG measurements are real. If they are (which I doubt), your car is badly broken. I have not heard of anyone getting that kind of mileage on a Prius.

 

You can set up Fuelly to accept text messages reporting your fill-ups. It then instantly calculates MPG, logs it to your account and car (you can have multiple cars on an account), calculates the running average and texts you back the results. It's very convenient and fast to use. I usually shoot a text right after I am done filling up and before I zero my Trip A and take off from the pump.

 

1) how many fill ups have you done?

2) have you looked through the 'why are my mpg's so low' sticky?

3) there are many things on a prius that can reduce mpg's such as tires/pressure, oil level, 12v health and etc.

 

I also was freaked out about the fueling at the gas pump stopping at a low amount. I have found that on fuelly even if I feel I have filled the tank I mark it as a "partial fill-up". Then, on the next fuel stop I mark it as a full fuel-up. This makes fuelly use 2 tanks for my next recorded average. This helps smooth out the fuel bladder variability. I bought my Prius in early February and live in Michigan. My first or second fuel-up on a tank that was low stopped at 3-ish gallons! I didn't know about the bladder and thought I had an issue. Prius Chat has been a wealth of information that has helped me during my steep learning curve.

Finally, and again if you have an Android device, you can download Hybrid Assistant. It is a wonderful app. One feature is a "HV battery check". It shows the voltage and resistance of each battery module. Another app is Dr. Hybrid (formerly Dr. Prius) that does even more battery checking. But, I believe this requires a purchase whereas Hybrid Assistant is a free app.

Good luck with your fuel economy issue. I still have high hope that over a few fuel-ups yours will sort out nicely.

Paul.