tyre size

I have 265/75/16 on my defender 90, I've just bought a spare , Do I have to match the diameter size ie.75 for the spare would a 85 cause damage and wind up the transmission many thanks John

For the MOT the tires have to be the same each side of the axle. For a spare it is fine [ like space savers in cars ] There is no problem with wind up, preventing this is what diffs do unless they are "locked" Only the centre diff can be locked on a standard Defender
When running different size tires on the same axle the diff will just think it is going around a corner all the time.

Ideal thank you