Stock is 15" for the bottle caps and 16's are noticable but not much, they still look stock and not near as good as 17 or 18s. Also if they are a Z3 you need to see if they will fit. Check the offset and bolt pattern. I am not sure what it is for a Z3 but I am sure you could find out. The E36 318 should be 5x120 and somewhere around 38mm offset. Post some pics and show us what they look like.
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