Unhook the speedometer cable from the transmission and the instrument cluster in the dash. Take the actually metal cable out. Clean it with solvent. Re-grease with "lithium-based" lubricant. Re-insert.
Also check for fraying. If so, replace cable.
Stop driving it
Replace the speed sensor, usually mounted on the transmission bellhousing.
1998 Dodge Caravan Speedometer Has a second connector attached with a white pigtail. What is that used for and will it operate without it?
buy another one from a breakers yard
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.
A 98 Civic does not have a speedometer cable. It is an electronic speedometer. So if you are having problems with it not working the most likely culprit is the vehicle speed sensor (VSS).
You need to determine if it's the speedometer itself that's faulty, or the vehicle speed sensor. If the speedometer is faulty, you'll need to replace the instrument cluster.
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I need to fix the odometer and speedometer in a 97 ford explorer
Send to a speedometer repair shop or replace speedometer head