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		It's circus really, the RTA, found you just don't tell them anything, find an understanding mechanic for the annual inspection and don't do any mods that are gonna make it stand out too much in traffic so you don't draw the heat. Apart from that do whatever you like, we're over-governed enough as it is. 
I've probably got illegal tyres on mine too but it looks like every other series with wide wheels so they don't bother me. :wink:
	 
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		This is the greatest resource for fitting tyres to a Disco2 and calculating differences in tyres 
http://www.disco2.com/howto/offroader/tyresizes.phtml
BTW, In sydney I've never been questioned as to the fitting of my 33" tyres on the Disco, and thats even when I goes in with the Simex tyres. 
That does not make it legal though. 
Another interesting point is that if you go to a number of tyre manufactureres web sites, you will see that tyres of the same designation will vary by more than 15mm, and then if you include wear rates, that only leaves 7.5mm from the OD. Tyres like Coopers have 15mm tread depth.
	 
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		That's one of the good things about the Defender, the 235/85/16 size is very nearly 32-inch and the largest diameter wheel of any stock 4WD to the best of my knowledge.  So I've got 32s and it's legal!