530i Fuel Tank Capacity and Reserve Tank

I searched for a couple of hours, no luck on this question.
I read the manual, very little info.

So, I think the Total Fuel Tank Capacity is 18.5 gal (aprox)
The manual says the Reserve Tank is 2.1 gal

Q1: Is the 18.5 gal the absolute total? i.e. I can only load a max of 18.5 physically into the car; or I am really filling up to 20.6 gal, including the reserve tank.

Q2: (Probably stupid question ) Why do we need a reserve tank? Is there a mechanical advantage? How does it work? Why should the car even have one with modern sensors that call tell the current capacity?

Thanks for your comments!

 

Its 1 tank. It just says "reserve" since the warning goes off when you have 50 miles left before you're empty off the TOTAL of 18.5 gallons. Dig it? =p

 

The lowest I ran my car down to was 50 miles and the gas gage needle was halfway between 0 and 1/4. I took about 15.5 gallons to refill. Based on that I would assume that when the needle hits 0 or darn close, you are truly out of gas and the warning lite come on before the needle hits 0. Most cars I have driven still have gas at or real near to 0.

 

Moral of the story . . . . .don't let the fuel get below a quarter tank.

 

Fuel gauges are very inaccurate, even on small airplanes. We have all heard about people running out of gas on the interstate, and: bingo, they get creamed from the rear.

 

+1

 

Lowest I've ran mine was when it said it had 6-7 miles left in it, I didn't have time to fill up as I was rushing to do errands all before trying to go to a memorial. Well when I filled up, it took 17.XX gallons, I want to say mid to high.