I read the post above. I forgot all about those adapters. If you look on eBay under Stewart Warner Speedometer Adapter you will see all kinds of them for sale. You unscrew your speedo cable nut, stick the the adapter on in its place and then stick your cable back on the adapter. It would take about minute to install.
Check this place out they have all kinds of speedo adapters. They claim they can make up any adapter. Its probably not going to be cheap. I remember buying those Stewart Warner speedo adapters 35 year's ago. I can't remember what phone company vehicles we had that came with these adapters from the factory? After a few years they would start leaking puddles of ATF and we would need to replace them. I'm thinking it was the Chevy 4x4 Suburbans with automatics back in the late 70s or early 80s, it's been awhile? They probably needed the adapters because of the 4.10 gears. Those adapters were really expensive from the dealer even back then. It had to be the Suburbans because I just found these photos of a NOS GM adapter for sale. As you can see it has the ratio stamped on it. The speedo shop could make just about any ratio you needed by changing the little gears inside.
Speedo adapters
http://www.txinel.com/speedo.asp
I don't know how you change the actual speedometer calibration? A spinning magnet is what usually moves the needle. Maybe you change the strength of the magnet? That's all going to be a lost art now that gauges are all electronic. There was a Stewart Warner Calibration machine on eBay awhile back. It was really neat and I wanted it bad, but it was too expensive for just a piece of garage art that maybe I would use once in a lifetime. That is the only one I've ever seen. It was a quality piece of equipment. I can't even find a photo of one on Google to show you. Now I'm upset, I should have bought it!!!!!