Well replacing the speedo gear will correct the speedo being off roughly 10mph but I have never herd of it shooting way up like that and not correcting itself until you slow way down..
I would start with the speedo gear.
I know. I had a set of polished 18's on my 94 years ago and it took hours and hours of polishing and waxing all the time..
IMHO powdercoating is the way to go (or chrome)
Unfortunatley all you can do once they are polished up nice is keep a couple coats of wax on them. That will help.
OR
You can get them powdercoated. (That's what I would do)
LOL, its a pioneer. I love it but it don't have GPS in it (it is touch screen though)
The SVT coin delete was on ebay a while back. I can't find them anymore though.
GL man!
-Rob