mine gives 11km/l and it was bought brand new....its been giving ~11km/l since new.
Its not a very efficient car at highway speeds. The best efficiency you get on it is at ~60km/hr, at that speed in a straight line, you get ~20+km/l. Its meant for city driving...
The car is underpowered.
But still, get it checked/tuned. Maybe something with the spark plugs etc...?