1991 B350 Speedometer problem HELP

Hello, I am driving a 1991 Dodge 1 ton van that has a speedometer problem that I cannot figure out. When the Ignition is turned to the run positinon the needle on the speedometer goes all of the way up and
stays there until the van is shut off. The Transmission shifts fime still and does go into overdrive (from what ive been told it will not go into overdrive unless the speed sensor is working) and cruise control works fine (also told that if the speedometer isnt working it shouldn't go into OD. Could this be a bad speedometer? Does anyone know how to test to find out if it works correctly?
Any and all help would be greatly appreciated.
Thanking You in advance,
Chuck
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