Cyee, I have an 18i as well, and yes it is heavy on fuel. I assume you have auto? - I do about 50/50 highway/city and my average is now about 10.5/100km. I know it's a pain, but what will really help is if you use Steptronic in manual mode. What irks me is that in full auto it hangs on too long on low gears when going up hills - sometimes won't change up till 4000rpm or higher.
My situation is a bit worse as I am at high altitude.
The manual mode works quite well.