Bladder? Flashing pip? You guys have me scared

It might sound stupid... but how much does that 5 gallons of gas weigh? They say you shouldn't have stuff in your trunk you don't need because it will impact your mpg, I would think that the weight of gas would have the same effect, the weight of everything in the car adds up. Including my gut... and my trunk. Ok, time for a diet. Next week ;-)
We wouldn't be discussing this in any other car, too bad ours reads out mpg to a 10th of a mpg.

 

The owner's manual says that when the last pip starts flashing you have 3 gallons(or less) left in the tank. Last weekend we were about 15 miles from our home off-ramp when the last pip started flashing. The car being pretty new, my wife looked at the manual, said 3 gallons remain, 48mpg, ergo 140mpg(max). 10 miles later everything started flashing, the ICE stopped running, I pulled over. We got off the fwy to a parking lot on battery power only, shut it off, started to call for gas. Then I tried turning it back on, the ICE started, started charging the battery, but all the warning lights were still flashing. Went straight to the nearest gas station and added 10.5 gallons. Everything normal. Obviously, I will be filling up before it flashes from now on.

 

Buying gas for my Prius is so much fun compared to my previous car that I do it whenever I get the chance. I love to pull in next to some big beast SUV that has been pumping for awhile, fill up and leave before they are done. The open mouth look that I get from them is priceless. I usually fill up with two or three pips left.

 

approximately 6.1 pounds per gallon