I have the same city in pretty nice and stock condition. I do have CNG but my intake is not blocked and timing is tuned for running on petrol. I am running Bosch 2 Pin plugs and stock wires with liquimoly MOS2 10W40 Oil. The max i get with light foot without AC is 14km/l. Even 14km is great average for a 1.3.
This model of City is OBD0 so it does not have O2 sensor and few other sensors with newer OBD2 cars have. It still runs with distributor and common coil unlike newer cars where the spark to each and every cylinder is controlled by the ECU and mixture output is monitored by O2 sensor.
18km/l is just too good to be true specially with Auto transmission.