No, my alloys are 17" the space saver isn't. What's important is the rolling diameter. Use
http://www.tyresave.co.uk/tyresize.html, put 225/45/17 in the old tyre size and your tyre size in the new, eg 215/55/16. If the two are within a few % of each other it'll be ok as a space saver.
I rolled past a v50 the other day and noted they're the 5 stud, I presume the dia is the same.
Top right of the page is where my pm link is, don't know if you need minimum number of posts.