Ok I am wrong,
It appears the tyres I am talking about may have been non standard. Just looked at some pics, the original Michelin tyres on the Disco were 235/70R/16 so around 29". The second set on were substantially taller but have not been able to find a pic that I can see the actual size and they did not foul, but I think were 235 or 245 /80. I do remember talking to the Tyre Power guy about fitting slightly larger tyres, but that was 12 odd years ago, but I thought it was in the standard range.
The ones on the Defender 235/85R/16 which we know is standard.
The RRC most likely had a tyre size change well before I bought it and I need to find a pic that shows the tyre size. So obviously not original size. I dont think they were 235/85R/16 but overall were the same diameter, give or take a few mm. Possiblly 245/80R/16 which would mean less than 1/4" difference. Maybe as the RRC ones were 60% worn and the Defender ones new were real close in overall diameter?
Did not know the RRC ones were not standard fit.
Something to watch out for though I suppose.
I know when I did put new tyres on the Disco it changed the fuel economy slightly from 10 or 11 to 9 or 10 litre per 100kmph, maybe due to the increased OD. Can not say I noticed any significant loss of power though.