Overall very good car but poor mileage

Pros - Its been now 2 years I have been using tata zest car (xm petrol) . Look wise its very good, its stylish, comfortable and spacious. Performance is also good, EXCEPT MILAGE. Cons - Poor mileage, My car is giving mileage 8/9 km/l in city with AC and on Highway its giving around 10/11 km/l with AC. Its very poor. On the document this car has mileage of 15 km/l in city and 18 km/l on highway. I was expecting at least 13 km/l in city and 15 km/l on highway with AC on, but its too low than expected. I would recommend people to go for diesel version instead of petrol.