I've read several old threads on this and am unsure if my car's problem is VSS(Vehicle Speed Sensor) or Speedometer. Like others, my car's speedo stays at about 18mph and then begins reading correctly for a while and then may drop back to 18mph. The odometer stops turning when speedo is stuck but I haven't checked the cruise control when speedo is stuck at 18. I know it still works when speedo is moving. I think problem started around the same time I replaced thermostat and had problems getting new gasket to seal so leaking A/F may have damaged sensor?
I don't get any CEL. Any suggestions on how to correctly diagnose?
Thanks, Charlie

 

Check the cruise control like you mentioned in your post. If the cruise works, then the speedometer is the problem.

The sensors near your thermostat will not affect the speedometer reading. You can unplug and clean out the connection to the VSS using some electrical connection cleaner. It may have gotten some coolant in it.