It is either the speed sensor or the wiring. Have it checked out.
Cruise will NOT work if the speedometer has a problem
The speedometer sensor relates back to or through the ECM and to the cluster in digital or analog out put.
If the speedometer and odometer are both not working, I would blame the speed sensor. If only the speedometer does not work and the odometer does work, the cluster is bad.
what causes the speedometer not to work
There may be several reasons whey the digital speedometer might not work. The first is that the LED light may have burned out. The second is that the fuse may have gone out. It also might have a bad speedometer cable.
Is the TCM working normally and throwing no codes?
If the digital dash lights up but shows no numbers for the speedometer on a 1989 Thunderbird LX, it is possible that a fuse is blown for the speedometer and the fuel gauge. There is more than one light in the dash for the speedometer.
speed sencer
The odometer has failed
This is an electrical problem that happens with a lot of Caprice classics with digital spedometers. My speedometer works about 1 day out of the month and the will cut off the other 30 days.
digital cluster may need replaced
If the speedometer doesn't work at all then there could be two causes. The first thing is to check the needle it may be loose on the spindle, secondly there is a relay from the fuse box to the speedometer.