My two cents. using swift Automatic variant since 2017.
highway / motorways = 15 to 16 with AC
city = 12 to 13 even plus.. with AC
engine oil = 5w40 fully synthetic
more importantly, its a 1300cc Plus engine ..... not 660 or 1000 so do not compare it with low powered engines and their mileage. one cannot expect high mileage in city...
it gives you instant power and boost to enhance your driving pleasure, if someone concerned about mileage above 16 / 18 etc. then better to buy hybrid / electric car
or low powered engine car.
last but not least, always drive gently to improve mileage of your car.