speedo question?

so I just replaced the speedo cable assembly ( new ) and speedometer, ( used one ), in my sons 74 Duster, it has a 904 trans now as soon as the car moves forward a couple feet the speedo starts to move quite a bit, like 50 mph in two or three feet then when I stop the speedo stays put and when I go backwards the same distance the speedo goes back to zero, whats wrong?, maybe a bad speedo?, if so any way to test it? thanks