The problem of Fuel gauge not rising up if you put less fuel in it happens with my car too. However, my solution is to "keep the switch on" (turn off the car, and turn the switch back on -- don't start it) while filling the car. This way, whenever I fill up, even if I put 500 rupees of Petrol, the needle jumps of immediately.
As far as trip calculator is concerned, something is wrong with your car if you're getting 9/10 Kilometers per liter. I never got this average, ever with my car.
I can put 1300 rupees of Petrol in my car in Rawalpindi, and go to Peshawar and come back without refueling. I do this on every second weekend. This makes the average around 21 Km/l. This distance includes the distance I travel within the cities to reach my home (after I get off the motorway). So regardless of what the computer is showing, the fact that I can drive precisely 400 kilometers in 1300 rupees implies that I am getting an average somewhere close to 21.