13 litre per 100 km is actually not that bad. I have a diesel and if my avg speed is 26km/h I would be lucky to be under 10 litre per 100km.
The problem is the engine is woefully underpowered. Surely you have gone for a test drive? Did you research what the 320i's fuel consumption is using the same engine? Being underpowered also mean you are pushing it all the time just to keep up a reasonable pace and hence detrimental on fuel consumption.
When did you get the 18i?