There are a lot of things it could be other than the Speedo head unit.
You have two gears in the gearbox. But ill fitting cable is a likely culprit. That's not to say that the speedo head unit is faulty but I would be looking at the whole thing (even the newly fitted cable).

If you want to test your Speedo, take it out and using a piece of speedo cable and a drill you can beach test it.