I bought a city 2007 brand new and used it till last year for 59000 kms. It used to give me around 13 km/liter in city (Islamabad) driving with AC. If I took it to Islamabad highway and stayed till 100 km/hr i.e. 3000 RPM - average used to improve to 16 - 16.5 - 17.
On motorway the avg dropped to 14 - 14.5 if I drove it at 120-125-130. I never managed to drive it at 100 on motorway - it is hard - but if you do so I am sure you can achieve 17-18 km/liter.