Altis has highly sophisticated engine technology. In fact almost all new cars give an average of 14+km/litre.
The mileage of a car depends on more than 20 factors which are likely to vary from one car to another. For example.
Type of engine oil
Condition of engine oil
Car tuning
Air filter condition
Tyre pressure
Weight of car
Length of route
Amount of traffic
Heavy/Light foot acceleration
Windows open/close during long drive ( Creates drag and increases resistance)
Clean car with smooth surface or dusty car with rough surface.
Uphill or downhill road.
Any car body modifications
Allow wheels of car which are heavy.
the list goes on and on and on...
In conclusion. Mehran ideal average is 16km/litre as stated by Suzuki itself. This is all one can desire from this car. Anything more is either extrapolation or miscalculation