Thanks to all for your responses! BTW, it turns out that the
fewer teeth the speedometer driven gear has, the higher the speedometer reads!
So here's what I did: I wanted to avoid taking the dash apart, so I tried switching out the speedometer driven gear at the trans (attached to end of cable). First I tried a 17 tooth gear, and then the speedo didn't work at all! So then put in an 18, and that made a noticeable improvement. The reading is still slow at 0-15 MPH, about right at 20-45, and a little high at 50-65. Not perfect, but better than it was before. I would say going from 19 teeth to 18 increased the reading about 5-7 MPH average.
I like the fact that this is a responsive board, and that so many people replied to my post!
Here are two more pictures, and a link to a web article featuring my '58 and a '57 Ford:
https://www.curbsideclassic.com/blog...-the-late-50s/