When calculating the mileage, also keep the quantity of the petrol you get from a particular petrol pumps, most will give you 0.7 for 1 litre or even lesser. If you keep an eye on the fuel gauge after you fill from different petrol pumps you will know the real story.
If you do not switch off your engine while waiting at a signal (more than 60 secs), you will never get good mileage. Also waiting in the car with AC on in the parking lot while the family is shopping destroys the good mileage dream.
If you are in habit of running into a red light and then on green light launching the car as if a race is starting, you will never get mileage.
If you are in the habit of overtaking every car in front of you, you will never get good mileage.
Most people would never care about such things and then say gaddi tel buhat khaati ha.