It's not the wheel diameter that impacts the spare tire bay, it's the width.  I run 16" alloys on the ground and have a stock steel 14" for a spare.  Same diameter but the steel one is narrower and only used to get to repair shop.  Just make sure you have a set of lug nuts for the steel as I believe the alloy nuts are different and may cause a problem, but I have used the alloy nuts successfully, too, for short distances.   Good luck.   Dan L.  89 Country Homes Camper 

    On Sunday, January 8, 2017 8:44 AM, Dan N <dn92610@GMAIL.COM> wrote:
 

 in case you won't feel comfortable with the tight fit of the 15" spare, you
could get an oversized spare tire carrier

http://www.van-cafe.com/home/van/page_348_129/oversized-spare-tire-carrier.html