@Baaze,
there is no other reasonable way to find out, than to take off your front wheel and:
-like you wrote there is in this "speedo drive gear" a small sprocket which (simply said) connects the drive with the odo cable. Check whether those parts are rightly positioned, and your black bigger outer gear part is hooked up with its nose at the fork.
It helps if you "glue" the small sprocked in with some grease during assambling.
One more reason could be your odo- cable not far enough in its sheath, down there where this #screw is located at.