I have a 92 dodge 2500 cargo van, 5.9 w/518 with a speedometer issue. It has always read accurately in the past but recently reads a fraction of the actual speed. Say I'm running along at 70mph, it reads 20mph. The needle does not bounce around, but will hold steady. What would cause this? Thanks in advance.

1) The speedo cable is dry and /or binding in the housing.
2) Cruise control? Does the speedo cable run through the cruise control diaphragm?
3) the speedo gear on the trans is dead (or near dead) is there an electric pickup on the speedo gear housing?
4) The speedo head is dead or near dead.
5) If there is a speedo correction device, it could be bad.
Gene

does the 92 have the VSS speedo gear housing with a wire/plug at the speedo gear? long or short speedo pinion gear shaft?

may need the unit replaced.

Does the odometer record accurately?