standard spare wheel is i believe a 205 85 16.
Whos is?
Standard tyres on a RRC are 205/80R16. Some 235/70R16 will fit the spare wheel well on later cars if they are road biased. I find AT and MT will not.
A 235/85 will not even go in the car let alone fasten up to the spare wheel bracket.
Jonny, your problem, other than the lawm is the centre diff may not thank you for running differetn sizes on odd corners. You take the chance.
My advice would be if you must then put the samller wheel on the back so that the front axle turns slower until you come to a corner.
With a vicous diff it's not adbiseable to use different types of tyre that are nominally the same size, or even 2 worn out tyres and 2 new ones.