Am I the only one who has a full size matching spare mounted to the vehicle? If it is not a matched spare then you cannot rotate the tyres around as
@Flossie mentioned. I actually have two spares because it was recommended, although completely unnecessary, for the trip to Morocco, one lives in the garage one lives on the back of the 110. Similarly with the series (also two spares, one on bonnet one on rear bulkhead) all the wheels and tyres match. This allows them to be rotated around so the spares are roughly the same amount of wear as the rest of the tyres and when replaced it is not a brand new never used tyre which is too old to be any use. I know it is easy for me as I can swap pairs, but you can do the same thing with a singe, move the spare to the front, front to back, back to spare etc. This ensure more even tyre wear, and allows maximum usage and mileage out of a set of rubber.
p.s before any one complains that rotating them is extra work, if your landy is like mine the wheels come off for maintenance/repairs often enough that it is not a special job, you just swap them at this point.