I just completed a 4000km trip through central QLD in my 4sp, 3.5 TJ wagon. There was only two of us on this trip but the back was loaded up with allsorts of camping paraphernalia. We travelled in "chortle" mode which means a constant 90kmh with no aircon and returned an average of 7.7l/100km which is 850-900km per tank. Other factors were flat and straight roads, fresh engine oil (including wynns engine treatment....thanks Mal), mostly 91 ron ULP (trip was on a fairly tight budget), the tyres were 32psi cold (I thought they were higher but checked them when hot.....my bad), and windows open. We passed through numerous towns including some big centres like Townsville and Rocky and were stopped for prolonged periods 1/2 dozen times at major roadworks but even with that I would say 95% was open highway. All in all am very happy with the outcome.