Over all good but lack of milage

I purchased celerio zxi amt optional on 4 September 2017. It is a good car. But it has a lack of millage. Company shows it 23.1 kmpl. But it gives only 16 kmpl. Space and boot space is good. Seat is comfortable. It is top model, but siren system is not available. We can not fit anti theft alarm system in it.