It seems to me that driving style has a lot to do with it. I recently bought an auto 2008 JDM alto and it is constantly giving me over 16 KM/litre and i have taken it up dirt roads as well.
My advice - get stock dimension tires and regularly check air. Clean air filter and most importantly , drive with a light foot. No sudden acceleration, no sudden breaking. Enjoy the ride... hopefully this will give you some additional KMs per litre.