Tata Altroz Service Cost

I want a car for family so altroz fullfilled all the desires yes engine is little bit not much peppy but it can be ignored when built and other space is good service cost clock 7 k kms and not getting any problem.

I have driven about 18000 kms.Mostly in city.The vehicle is nice to ride especially on highways.Initially before 2000 kms i had a chance to drive on lucknow -agra expressway, I went at the max speed of 80-90km.Altroz gave me reading of 24kmpl and that's the best till now for me.After that the average mileage was 17-18 kmpl as i drive mainly in city (not much traffic).But after the last service, The mileage suddenly went down to 13.5 kmpl (avg of approx 14 kmpl).Also on highways it's 18 kmpl max.(previously on highways was usually 21-22kmpl).Please, Anyone who could give the exact reason for this??thanks for reading.