It only took 6 gallons this morning...

Did all 10 pips then light up after the fillup? My tank is really picky about the pump I use to fill up and how I do it. It often cuts off early and the tank isn't actually full. Highly irritating.

If all 10 pips showed after the fillup, you probably need to recalibrate the fuel inclinometer sensor. I had to do this when I first got my Prius and it did help the accuracy of the fuel gauge. See this post for the procedure: Gas gauge didn't show empty

 

I would not worry about it as the G2 Prius uses a bladder in the fuel tank. In colder weather I believe the bladder stiffens not allowing as much fuel. My guess! H

 

Is it really cold where you are? The difference in capacity (due to the bladder) can easily be over a gallon different between summer and winter.

 

You do not list your location, but if you are in North America somewhere, then the gen 2 (2004 to 2009) Prius has a 'flexible bladder' inside the gas tank to reduce gas vapors. Sadly it is more flexible in hot weather and less flexible in cold weather. So in August I might get 11 gallons on a fill up, where in December I might get 7. (I am in Mississippi, so many areas will get colder than I do) Later afternoon will allow you to add more gas than early morning.

So no, there was not 'hidden gas' it simply holds less the colder it is. Fill up at two pips and you never need to walk several times to your car bringing it more gas. (The gen 2 will need 3 gallons to 'see' that you have added gas)

 

I have not heard of any fuel gauge recalibration, but in my case it really held less in winter. A lousy time to walk for gas, two pips!

 

I am in MN, too. The effect of the cold does have an effect -- probably a gallon or maybe two. However, in my case, doing the "recalibration" (it is really just resetting the inclinometer) did have a positive effect on the fuel gauge accuracy.

The degree of inaccuracy you are seeing (expecting 9+ gallons and only filling up 6) is similar to what I used to see. Now mine is much better. You don't have much to lose with doing the procedure I linked to above, it won't hurt anything....

 

My 2005 has the same issue. A fill up in winter is only 7 gallons or so. In summer it's 9. Basically fill up 100 miles different. No big deal -- my previous trucks and daughter's Jeep are lucky to go 250 miles on a full 20 gallon tank. I get 300 miles on less than 7 gallons.