Joe, I've never been into a 122 speedo, but I've worked on a few from 1800s and I imagine the principle is similar.

In the 1800 speedo, the cable drives a bar that's magnetized. The bar is contained inside a sort of flat dish, and as it spins, the magnetism pulls the dish along with it. There's a spring on the indicator needle that opposes the dish rotating -- accuracy is a matter of spring tension, RPM of the bar (determined solely by the drive gear in the tranny), and the distance of the bar from the dish.

The bar and input shaft are held away from the dish a specific distance by the worm gear that drives the odometers. Over the years, the worm gear slips on the shaft very gradually (under pressure from the cable, apparently), and the bar moves closer to the dish, increasing the effect of the magnetism and cusing the indicator to read high.

Eventually, the bar actually hits the dish and makes a clattering sound, and the speedo reads very high, and often bounces as well. It's not hard to reposition the bar shaft, but I have never found a way to make the worm gear hold it there for long after it slips the first time.

If the 122 speedo is built the same way, you'll better off transferring the good-looking front panel, etc. onto the mechanism that works right.