The trouble with words like center is that they are far too imprecise. One could argue that as long as an object is placed so that its outer edge is equidistant about an axis, that object is a "center" + <whatever the object is supposed to be> (cap, cover, plate, etc...). By that definition, my wheel covers are "center caps".

Ironically, the term hubcap is to me the only one that is explicit enough to work in this instance but it's too late for that term to be redefined.

Here are the photos. As you can see, I use what is now called a wheel cover to great effect.