While installing the rear tire on Razzo last night, I found the nut to go further than it should and, when tight, the rear tire would not turn. Something was wrong. I knew that a spacer was missing. I checked the Miata to see if it fell off while transporting the wheel. Not there. Sarah and I went out to dinner and the issue dominated my thoughts. When we returned home, I found the part on the garage floor, but couldn't figure out where it went. I took off the swing arm and placed it between that and the wheel, both ways. Did not fit. So I took off the wheel and realized that it went on the spindle behind the wheel. It fit like a glove. So back together it went. I have changed rear wheels several times and this piece never came off. It must have come off with this removal and I didn't know. Now I know.