speedometer

So when I got the car the speed cable wasn't connected. Tried connecting and it bounces up and down and makes a clicking sound. I keep 5 miles under the speed limit according to the speedo but I know that it's then the speed I'm really trying to go as I've checked with my phone's speed sensor and my lady following me and by the popos speed radar thing they put on the side of the road at times. What I'm wondering is do you think it's the cable or the spedometer or both. I'd like to fix this soon. But it may wait til I rebuild the blocj