A 15inch rim (which usually has a 15inch tire

) would need a hubcap designed for a 15inch rim.
A 13inch rim (which usually has a 13inch tire

) would need a hubcap designed for a 13inch rim.
A 17inch rim (which usually has a 17inch tire

) would need a hubcap designed for a 17inch rim.
There's a pattern up there BTW
