Speedometer calibration

[Riocobravert]

I just had my rearend redone it was making noise. I had it put back to near stock (3:31). I made the assumption that the speedo gear had been changed to a 23 tooth to compensate for the earlier 4:10 gear change(before I owned the car). When I went to put in the 19 tooth (pink) gear I bought I was surprised to see the one in the car was a 19 tooth (pink) gear. I've been reading about speedometer calibrators, but they don't seem to apply to my 1997 Cobra. Some info says this is all mechanical, just the trany driving the cable direct to the speedometer. Can anyone help me understand how the speedometer was calibrated for the 4:10 gears so that I can get it back in a similar manner?

[Riocobravert]

So I went and bought a 16 tooth today, as small as they make according to the Ford parts guy. That got it closer, it's only off by about 6% now, speedo says 56 mph at 60 mph. Any ideas how this was changed for the 4:10 ratio. I'd really like to figure it out so that I can put it back the same way it was changed.