2002 Chevy Cavalier Sppedometer... runs about 60 mph high o

We have a 2002 cavalier that has a speedomter issue. It started out as a problem mainly when the roads were very wet, but now it is that way all the time. The speedomter basically reads 60 MPH while the car is standing still and goes up from there. Sometimes it doesnt work at all. Every once on a while the speedo will work intermitantly for a little bit. What would be the issues here? I know there is a sensor for the speedo but not sure if this is the problem or not. If the sensors is bad, how do i find it?

Thanks