Fuel consumption depends on so many parameters and to a great extent depends on the driving habits of the user. Normally the Engine will deliver power at a very economical fuel consumption in the following bands -
0-20 Km/Hr - 1st Gear
20-40 Kms/Hr - 2nd Gear
40-60 Kms/Hr - 3rd Gear
60-80 Kms/Hr - 4rth Gear
80 Kms/Hr and above - 5th Gear
Above data defers case to case
Secondly also even if you fill the tank full, and drive the car till the fuel tank is empty, the pattern of driving i.e long distance/short distance stoppage contribute to the performace of fuel consumption.
Condition of Air filter, Spark Plugs plays a very cital role in this
Additionally you can get the consumption checked accurately by fuel econometer which may be available with the dealers.