Dear Bro,
Better you do a long run on a flat route(Like anuradapura/polonnaru) and have a idea of fuel consumption then you could have a better clue than running in town. City limits traffic congestion is varies you can't have any reasonable idea. As an example I daily come to work in the early morning from malabe to colombo and never face any traffic blocks but gear changes rapidly due to color lights , inter sections and some other factors. my car is 1.8cc and does 9.2km/Lt(i am bit hard on the paddle). When i went to kurunegala recently it has done only 10km/Lt, There is no signifigant change.