The best way to calculate the fuel efficiency of the car is Tank to tank or full to full method.
To begin with, fill up the fuel tank up to the brim and note down the odo reading(say Akm); or set Trip meter to 0. Drive your car in the manner you really do and once the fuel meter tends to get near the empty mark(or even half), refill the tank to the brim and note down the amount of fuel(say Xlts) which gets into the tank. Also note down the odo reading at the time of filling(say Bkm).
Subtract this reading with the previously recorded reading(at the time of filling the fuel).
Divide the result(B-A) by the amount of fuel consumed(taken in when you filled this time).
That is, FE = [(B-A) / X]kmpl.
The figure obtained is your real time fuel efficiency