Fuel Tank Capacity???

Since I bought my demon 2 1/2 years ago, the fuel gauge as always read empty. I would maybe put 5 or 6 litres in at a time just to run it up and down the road. I always assumed there was a problem either in the sending unit or the guage or somewhere between, and thats why it never showed a level. Tonight i threw over 20 litres in it, and the guage came up just to the empty hash mark...what is the capacity of one of these tanks? Surely 20 litres would register more then that, wouldn't it? I am trailering it to the alignment shop in the morning and may try to fill it to see what happens...