I have found some documentation on some old non Chevy speedometers that are very similar. It talks about changing out the gear that runs the odometer to different tooth numbers. It also talks about how not to damage that blasted spring that is around the shaft that holds the needle. I wish I had known that before I took it apart and it did a slinky on me.
I am going to follow these instructions and see what happens. I don't know the orientation of the spring when the whole thng goes back together but I might get lucky. It won't hurt to monkey with it a little before I give up and spend $150 to $200 to a shop and have to wait a couple of months to get it back.
Gater

1948 Chevy 1-Ton 3800