I recently did a trip from the Gold Coast to Cairns and back with a wife, kid and car loaded to the hilt. The trip computer readings were fluctuating between 8.5L/100km and at one stage it was sitting on 8L/100km on the way back (must be because it's downhill). I had cruise control on sitting approx 2-3k's over the speed limit. I was sitting 2-3K's over as the GPS, I have been told, is more accurate than the cars speedo however every now and then I had to put the boot in to overtake some grey nomads and some 'Wicked' vans. I should have recorded the distance between tank fills, I got 660k's from top up with just under a qtr of a tank remaining but i don't pay for fuel so i didn't care.