I am wondering if this would be a gauge problem or not. My speedo reads way high and jumps around but the odometer and tripmeter work just fine (checked it with my gps). This sound like a magnet problem in the gauge?
Most likely the speedo head unit is junk. Make sure the cable is all the way in first, you'll hear it click when fully seated. Also, if you take the speedo out and turn it by hand it should smoothly go up in mph's, if it just bounces all over the place the magnets junk.