The manual for my E55 states something along the lines of "21.1 gallon capacity including 3.17 gallon reserve capacity."

Does that mean I have 24.27 gallons, or does that mean the TOTAL of all fuel capacity is 21.1 gallons?

 

seems like total including reserve is 21.1

 

If you analyze the proper grammar, what you're saying is correct. However, this is somewhat confusing to me. I would just say, 18 gallons + 3 gallon reserve for a total of 21 gallons. This is more straightforward to me.

 

So does the car warn you that fuel is low when the main tank is low, but you still have the 3 gallon reserve left? How does the car cycle gas between the main and reserve tank?

 

I am very sure that these cars only have one gas tank. The reserve capacity is basically a warning saying "start thinking about putting some more gas in me now, or risk running out of fuel in very unfortunate circumstances". Like in a very bad neighborhood, or out in the middle of the desert where your only companions will soon be vultures.