Stock Miata wheels are hubcentric. This means that the center bore of the wheel matches the 54.1mm outer bore of the hub. This doesn't make for any difference in performance, but it does make it easier to install the wheel. Once the lug nuts are torqued, the wheel will center itself on the tapered lug nuts. Put another way, these are a luxury as opposed to a necessity - on wheels with a 67.1mm bore. 

Many aftermarket wheels have a 67.1 center bore. If you want to make them hubcentric for an NA, NB or ND Miata, you can use these centering rings. Note that these will not work on stock wheels or with any wheel that doesn't have a 67.1mm center bore. 

Anodized 6061-T6 aluminum means no corrosion and no melting. The fit around the hub is left slightly loose to prevent binding on debris or corrosion, but they will center your wheel for easier installation.

Set of 4, enough for one car.