Light weight spare on X3

[Kerdual]

Recently brought a 2004 X3 3.0i Auto sport with only 39000 confirmed mileage. Very pleased so far but have query over  emergency spare stored under the rear. The car tyres, which I presume are as original are 18 inch, but the light weight spare is only 17 inch. Having read a lot of buff on the importance of the right tyre and consistency around surprised that the spare is smaller. If I understand the guidance correctly this could damage the transmission. Any others have similar? Any views or obs appreciated.

[X5Sport]

Very common.  The E53 Sport had 19" or 20" normals with a 17" spare which was speed limited.  It is the outer diameter that is key.

[Kerdual]

Thanks for the confirmation, as you suggested the size of the tyre has more depth so the overall size looks close to 18 inch.
I am pleased I have a spare rather than the inflation kit, but it's a great shame that the carrier is not waterproof. The spare suffers from water exposure and could do with a waterproof cover inside the carrier.