The fuel consumption depends on fuel octane, how much air pressure in the tyres, wind direction and speed, your speed and acceleration etc.
I have travelled over 1000kms on a tank a few times but only on solely highway driving.
The old way to check is to fill up and note the mileage or set the trip meter, drive your trip and fill up again, note how much fuel you put in and the mileage again, this will give you how much fuel you actually used in the distance.
With your reading of 8.2, maybe your tank can take more fuel to properly fill up.