What you are achieving right now can be termed as instantaneous mileage/consumption, instead of average mileage. Normally when people quote average mileage it means that the car has been run on the newly filled petrol tank for around 200+ kms, thus to get steady and averaged fuel mileage.
The greater the number of kilometres driven, the more the accurate will be the speed average.
When the car has recently been filled with fuel and you take your car to steady and light footed 60 km/h, then even a 2.4L car would give an average of 18+. But if on a recently filled car, you travel a distance of only 1 kilometre but do a lot of idling, it'll only show you lets say an mileage/consumption of 2.4 km/l, hence the term instantaneous mileage.
I'm not saying that 16+ average on 1.5 PT is not achievable, but when we talk about average then it needs a lot of travelling to properly give you the average.
