Just did the first 5 tyre rotation the new 285/70/17 Duratracs and in 9,000km the tyres on the road are down to an average 13.4mm tread depth in the centre. The unused spare was on 14.4mm. The outer tread area had only decreased approx 0.4mm average compared to the spare, indicating what I suspected, they were running to high so have dropped the pressure a little more.
Pretty happy with that!
Also after a lot of research and fact finding I have come to the conclusion that the average rolling diameter of most tyres is approx 96.5% of the actual outer diameter (which equates to pretty much the diameter of the steel belts). So I have re done my speedo / tyre / gear calculator to update the approx speedo readings based on 96.5% of the tyre calculated diameter.
Anyone interested can grab it from here -
http://files.seacomms.com.au/Diff%20Ratios%20V1.4.xls