Gas tank reading question

Last year and this year I drove the same SoCal to Bay Area trip, on close to the same days of the year:

Last year: 356.9 miles, 47.9 on MPG display, 7.67 gallons to fill at end, 46.53 MPG calculated.
This year: 356.5 miles, 46.7 on MPG display, 6.54 gallons to fill at end, 54.51 MPG calculated.

The lower MPG this year was expected given some extended crawling along in heavy traffic conditions on two sections of trip.

The fill at the start of the trip was at the same pump and probably consistent, but the fill at the end of the trip was at different pumps at different stations, and they could have very different shutoff points. There were two pips left on the gas gauge when I filled it this time, but there is just no way that the car did 54.51 MPG, it always delivers lower than the display shows. The gas gauge is now "full", but I think probably actually 1 gallon less than full.

I know that "how much gas is in the tank" is always a guessing game on this car, but ballpark ranges, how many gallons correspond to "full", and how many correspond to "two pips"? Ambient temperature at this fill was around 65F.

Normally I fill the car shortly after it gets to 1 pip, and that ranges from 8.5 to 9.1 gallons (mostly, usually at the same pump). Assuming it was actually full (11.9), then 1 pip is between 2.8 and 3.4 gallons left in the tank. Except it probably isn't really full when "full". But I don't have a range of values like that for two pips.

I didn't want to manually force more gas in at that pump, as that can be a bad thing if it really was full, but before setting off on the return trip perhaps I will go to another station and see if that other pump thinks it should add a gallon . I like the car to have a full tank on long trips, just in case.

 

Decided to go back out now while it was still warm, rather than do this in the cold when I leave tomorrow. Sure enough, a pump at another station happily added another 1.735 gallons before the auto shutoff. That makes the actual gallons used 6.54 + 1.74 = 8.28, and so this:

This year: 356.5 miles, 46.7 on MPG display, 6.54 gallons to fill at end, 54.51 MPG calculated.

becomes this:

This year: 356.5 miles, 46.7 on MPG display, 8.28 gallons to fill at end, 43.06 MPG calculated.

which makes a lot more sense. (Kind of disappointing MPG, but crawling in traffic will do that. Drove around LA almost entirely on highways in the high occupancy lanes before this trip and the display was above 49 MPG, which is about as good as this car ever does over a whole tank.)

This also tells us that the range for full on the gas gauge extends from 11.9 gallons down to at least 10.2 gallons. I think the range could easily go another gallon below that.

 

Agreed.

Is the 11.9 gallons cited in various places the most that could fit into the outer tank, or the most that fits inside the rubber bladder under optimal conditions? Because the capacity apparently decreases with temperature and bladder age in some unknowable manner I wonder if Toyota just punted and gave the volume of the tank itself, and not the bladder.