I've lost count of the people who ask me what MPG I get from my bike but I do know they were all surprised to find out I have not got a clue! I know how far I can get on a tankful depending on speeds,types of roads etc.If I was going to worry about fuel consumption,I would have bought a moped.
I always work on the assumption that if it has wheels or wears a skirt,it will cost you plenty so that when your wallet IS emptied,it's no surprise!