New mehran easily reaches 19km/l without ac in city. Change you filling station which fills exactly the same quantity of petrol which you pay for. There are some filling stations which will charge you twice the price of petrol that you actually get. Keep in mind, mehran ki tanki 1800 - 2000 main full ho jati hai.
If that is not the problem then get your car checked from suzuki center by indicating them this problem or get it tuned from computerized tuning center, like enercon
