How do I know how much gas is in a full (very full) tank?

I have a 1996 Pontiac Grand Am (6 cylinder 3.1 liter engine). I know that the owner's manual says that the tank holds 15.2 gallons, but this isn't really what I'm asking. I understand that the tank is supposed to be full when the pump automatically shuts off when you're refilling the tank, but from what I've read, the shutoff point can vary between gas stations, and even between different pumps in the same station.

You can always add more gas to the car after the pump stops, and on my car I might be able to add well over another gallon before it starts to spill. I try not to do that, but when you live in NY and don't leave the state very often, you're driving from NJ and close to getting back to NY, and gas is about 40¢/gallon cheaper in NJ, you want to stuff as much of that cheap NJ gas into the tank as possible before you cross the bridge or tunnel back into NY.

For example, if you have a 7 ounce. plastic cup and you're pouring water into the cup, I find it hard to believe that you would have to fill it to the top of the rim, just before it overflows, to put in 7 ounces. The 7 ounce point is probably somewhere near the rim but slightly below, but exactly where I have no clue. I'm thinking that a gas tank has a similar concept. I can't imagine that I have to get it to the very top to the point of overflow to put in 15.2 gallons, so if I did put in that much, how much gas is in my tank? Is it 15.5, 16, 17, 18, etc.? Another point is that when some gas spilled and I stopped pumping, that tank wasn't really full to the brim, since the space that the nozzle takes up displaces some of the gas. So if I pulled the nozzle most of the way out, maybe I'd get a little more gas in there. I'm not that nuts and would never try it though, just to save a few extra pennies.

The reason I'm asking is because I usually get about 20-21 mpg, maybe a little more if a lot of driving is on the highway. I filled it up to the point of overflow, and I drove 319.0 miles and I'm guessing I probably had about 2 gallons left in the car, maybe even 3 (the gauge is inaccurate but I know approximately how much from experience). So based in that, my gas mileage was 319 / (15.2-2.0) = 319 / 13.2 = 24.167 mpg, or if I had 3 gallons left, 26.148 mpg. The first number is possible but pretty high, especially because a lot of those miles were city driving. The second number would be pretty much impossible (when the car was new that would have been about right). So I think there had to be a decent amount of extra gas in the tank after I filled up. I'm guessing somewhere between 16.5 and 17 gallons, but I don't know for sure. Does anyone know the actual answer?