Salam,
yar the tyre "width" do affect the car's milage but there's something seriously wrong with your car's tuning. Come on yar, you are talking about a MPFi engine here with a capacity of just 1.0 litre. 10 kilometers per litre is undoubtly a big problem. There is some kinda inspection going on for hyundai santro by some korean guyz free of charge at their dealers. Take an apointment and rush there. Those guyz are concentrating just on consumption.