I remember several years ago I had issues with my speedo cable. The speedo worked just fine but it would squeal so loudly. The faster I went, the louder it got. So loud I couldn't even hear my stereo! It was horrible. I just went and disconnected the speedo until I had time to get it fixed. It was so embarssing.
It has been about 4 years since the repair and it hasn't given me any problems. Although my car seems to think that 68mph is actually 80mph. I'll let her think she goes 80 on the freeway though.... :p