If it is manual, remove the cable and grease it with speedo grease. If it is electric, it probably has to go to the speedo shop.
My 94 had the same problem. Had to replace cluster. Got one off ebay for $40.
Speedometer cable replace 1989 toyota
You don't as it has a chain not a belt.
There are several things you can do to repair your speedometer. You can replace the speedometer cable. You can replace the speed sensor in the transmission.
How do you replace a speedometer cable on a 1984 Yamaha virago xv1000
You may need to replace the speedometer fuse. The speedometer cable can cause your speedometer to stop working. The speed sensor can also cause the speedometer to stop working.
Replace the ignition wires.
You can fix the speedometer in your 1994 Oldsmobile 98 by replacing the speedometer cable. If changing the speedometer cable does not fix your speedometer you will need to replace the entire speedometer.
You might want to replace your VSS (Vehicle Speed Sensor) Check this out: http://autorepair.about.com/library/faqs/bl022i.htm
Remove the dashboard cover. Remove the retaining ring, from the speedometer. Remove the speedometer cable from the back of the speedometer. Reverse the process to install the new speedometer.
Send to a speedometer repair shop or replace speedometer head
There is no speedometer cable on that vehicle. The speedometer is driven electronically.