But actually I did do that a while ago. Back when my ODO was at about 10,000 KMs. I filled the tank to top in Peshawar, and re filled the tank in Rawalpindi, and my car got about 600 rupees of Petrol.
Lekin back then I also remember the reading on the trip calculator turned out to be 97% close to the mileage I got by calculating again. I guess both figures were around 20.5.
From there onwards, I developed a good trust on the trip calculator.
Basically, the trip calculator 'knows' how much fuel is being burnt. So if you start your car when its on a down stream and you have the hand break pulled, its going to show 0 Kpl. Don't put it in gear and don't gas it, just release the hand break. Now it compares the 'movement' with the 'fuel being burnt' (even if its idle) and shows you the kpl.
This to me seems to be a very sophisticated system. Also the fact that they have put this in a car should imply that it really is accurate. There is no other gauge in the car which they have put which could be inaccurate. I am sure a car company always thinks twice before putting something which has a chance of being inaccurate.