Please understand this that mileage is dependent on many factors.
1) Weight inside the car, luggage or people.
2) Rush time or empty roads.
3) How you drive the car.
There are the reasons beside technical stuff which impacts mileage.
1) How can you expect a car to do same mileage with 1 person sitting with the car in which 5 person are sitting.
2) Empty roads results in getting good mileage, may it be inside city or Highways.
3) How are you pushing your car, speeding then breaking or doing it little wisely.