If ever your old speedo inner cable breaks, keep it and make two tools out of it! One for the wheel, one for the speedo head.
Start at the wheel and work upwards.
Disconnect the speedo cable from the hub. If need be, chock the stand so the front wheel is off the ground. Put the speedo cable inner only in the drive of the hub, hold it, and rotate the wheel while trying hold the cable and stop it from rotating. You should not be able to stop the cable, you dont have to use loads of pressure. Now spin the wheel in the opposite direction and try again. If you can stop it in either direction, change the worn and gear within the hub.
Moving on, if you can not stop the cable, then connect the cable back up to the wheel. Remove the headset top, and again try to stop the inner cable from rotating while you spin the wheel. Again rotate both ways to test this. If you can hold the cable, it has either snapped, or the cable is not located into the hub correctly. Refit the cable correctly, or replace as needed.
Finally all should be working fine now, if it is not, it must be the speedo head itself To test this, we use a cut off end of a speedo cable in an electric drill. Hold the drill on a steady speed and test. If the speedo needle fluctuates, the speedo itself will need servicing and re calibrating. It may be worth a try of lubricating the mechanism with WD40, as new or old speedos can suffer from the mechanisms becoming sticky. If this fails, professional service is required, or of course replacement with a new one.
OK, with regards to the speedo heads themselves, there are at least four current manufacturers of speedos that we know of, as well as the original Vagilia (no longer producing Lambretta Speedos). Only one we have seen is any good, and guess what, thats the one we stock! Of the others, some are OK, some look pants but work well, some look ok but work very poorly. We cant help with what one does what, but in general the green faced ones, or the ones we sell work the best, hence the cost more to buy and for us to sell. The old saying you pays your money....................... £10.00 - £20.00 speedos, will give you £10.00 reliability and accuracy. Ebay speedos give so many troubles, its just not worth the risk.