The manual says 15.8 gallons. It also lists the fuel tank reserve as 2.1 gallons. Does that mean there's a TOTAL of 17.9 or is the 2.1 part of the 15.8?

I find if I let it run down well into the red area on the gauge, it takes about 15 gallons for a fill up. It seems like if I were to let it go all the way to the E, it would probably take about the full 15.8 as specified. Which implies to me that when the needle first hits the red zone, that's the 13.7 (15.8 - 2.1) mark and the rest on the red zone is the remaining 2.1 gallons.

Right or wrong?

 

I'm thinking that the 2.1 is from when the low gas light comes on.
I've filled mine up a bunch of times within a couple miles of the low gas light turning on. Just as it hits the red part of the gauge and it takes 13.6 - 13.8 every time.