What tire pressure are you keeping in all 4 tires and are you driving with light foot? I'm also getting 15km/L Avg in city, calculated by mileage between 2 full tanks.
My office commute is 10km, at day time there's lots of rush on roads and it usually takes me 30 minutes to cover that distance at average speed is 20km/hr, fuel average is around 14km/L without AC. On way back home at night, roads are clear and I get around 19 to 20km/L Avg covering same distance in 20 minutes.