I am puzzled - nothing new there - but I would have been saying the opposite of what storm99 has said.
The different diameters of wheel make little or no difference to the overall diameter of the tyre, as fitted by LandRover to the various models as OE.
I have on my FL right now a genuine LR cover from a 215/65 16 wheel/tyre combo that I no longer own, and it fits a treat on my bigger than standard 235/55 17 tyre.
An 18" wheel will have an even lower profile tyre but it will still have as near as dammit the same overall diameter.
The differences in tread width should make no difference if you have the stretchy cord type of cover edging.
Or am I speaking cr*p because the 'body-kit' style of wheel cover is totally different?
Come back storm and explain, please.