So how to know if it is 16 or 18 for sure?

The main dealer can tell you the exact spec your car was shipped with ... allegedly. Mine was allegedly shipped with a factory winch but I can find no evidence of it.

The early cars came with 16" 8J or 7J rims (using up old 7J stock) or 18" 8J rims Later cars just had 16" or 18" 8J rims. The rolling radius is the same. 16" are spongier from the higher sidewall so you get a more comfortable ride but 18" feel more positive on the steering. If the roads near you are rough or you do off-roading go 16". I have both and swap them over if going green laning or sometimes in winter when the ice and snow makes the roads worse than usual.
 

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