Erratic Speedo?

Hi, i recently replaced my front tyre which of course ment removing the speed sensor from the hub. I refitted the wheel and took it for a short run at vairing speeds and the speedo worked fine. The next day i took the bike for a run, something strange happened. As soon as the bike reached around 30mph (espically on the gas) the needle would drop to 0mph.

Can anyone tell me how to fix this or give me any advice?

The wee bits inside the sensor are prone to being broken easily. I reckon that's your problem. Failing that, if it comes back to life, and possibly starts jumping around (assuming needle - curvy, numbers changing constantly on a pointy), then check your charging circuit.

Mine used to start and stop working, found when I put the new mudguard i'd caught the cable between the headstock nd the botom tree of the forks which had crushed the cable a bit, pulled it out and rerouted it and now works OK

Cable trap and breaks are the most common thing, you might find that when you move the wheel you moved the cable in such a way that it now shows up easier. I just stipped mine back and soldered the connection, as it had got caught in the steering lock and cut one of the wires.

Got the front wheel off. As i done this, all the four square dog teeth fell off the speedo drive gear. Some how i've crushed them when i put the wheel on after fitting that tyre. S*#t happens!