If it's a later tank it should be 20 litres in total, including 3.5 litre reserve, the older ones are 18 litres.
I try not to let my tank go under a third full at any time though, and i wouldn't have a clue about mileage.
I think cruising revs should be around the 4500-5000rpm from memory(i could be wrong?), although they do have a highly strung engine that doesn't mind being revved, hence the reason it redlines so high.