I know this has been done to death for discoveries and I've certianly plenty of experience in making big wheels fit to the disco.
Now I'ev moved to the defender 90 (originally a 90 td), I'm wondering what the tallest tyre is that I can fit without arch mods or similar (she's got a 2" lift) - I'm not too worried about width as ISTR military tests proving 7.50s are better in the mud that any other size they tested...
However I've got a low box on the defender which means it maxes out at around 70mph, a bit bigger than at present may help with the gearing for every day driving.
Present tyres are 255/75/16s I think, basically the same rolling size as a 235/85/16.
Matthew