Interesting, just back from a trip away last weekend. Travelled just over 1100KM on the onboard fuel (two tanks totaling 145L), 4 in the car with it packed to the gunnels !!!!
That is a good deal of range without a need to stop or look for fuel. I normally bargin on 1000km on the outside to be safe (and carry 20L on the back rack - maybe 20L of water though if it is summer and I am closer to home).