My car, the R33 from Brisbane to Rocky (650km's or there abouts) on one tank (100kph most of the way in 5th gear) and that on a 300rwkw tune...
Its all in the way you drive it Mark, you should know that...
I filled her to full at Shell at Aspley, right at the start of the Bruce Highway and set out at 100kph through the night, arived in Rocky as the sun was rising round 6am and the gauge was just under a quarter of a tank. I filled her up in Rocky at the BP oppisite the Harvey Norman store and put in 57 litres... So thats 8.77l per 100km
The other car, did Brisbane to Forster-Tuncurry (just north of Newcastle) which is about 675-700kms or there abouts also on only one tank of fuel...
The good bit is the commodore has a electronic fuel meter in the car so you can watch you fuel as you go, and the best I could get it down too was 9.0l per 100km's which i personally think is great for a 6 litre V8 which has less than 5000km's on the clock and is still wearing in atm... The range on the tank when we started the trip said 840km's, we got a solid 700 before the fuel light came on and from there we could have done at least another 75 or so k's...
But in short its all in the way you drive the car...
Actually you should ask Mud, as he drove his skyline up to Mt ISA, im sure he would have got some great economy on that trip...