There is no speedo cable mate. The U13 uses an electronic speedo.
You will have to work out whether the problem lies with the instrument in the dash itself, the speed sensor on the gearbox or the wiring in between. Chances are it will be a wiring or instrument issue though.....
As a first step, try and unplug both ends, clean the connectors and plug it all back in again.
Cheers, Dan.