there is no "how to" that I know of.
This is a very large job for such a small problem. For $89 dollars you can get a GPS that will show your speed.
Also there are other things that can cause this.
Note: I realize on cruise control it will probably jerk around with this problem but still the cost of replacing the speedo drive gear would be very large if a shop did it, and it would be difficult for a DIY to get done.