poor mileage

I purchased this car in recently and drove 800km in all conditions (in city/ highway etc---). I am getting an average mileage of 5.4 km/litre instead of 12-14 which company is offering. Please check the mileage of this car before buying this variant. It is a petrol variant with an engine capacity is 1.4, not worth to drive this car if you get an average of 5.4/km.