I believe that your speedo needs to be recalibrated. The part that the cable goes into and is driven by the cable which is magnetized, which in turn pulls the "speed cup"to move the needle to the correct speed. Over time the magnet becomes demagnetized and doesnt pull the speed cup enough. Take the whole speedo to a speedometer shop, they will take it apart, service and recalibtate it. This isn't something you can properly do your self. I hope this helps.