... japp!--> " The two tangs should meet and that is zero mph. "
The spring is pulling it back to zero but Yours stopps @ 15mph AND can be moved to 0mph!
Go and get a reference speedo and check them with a drilling machine at the same RPM!
Set the needle to 0 MPH and check the redings with the reference speedo, if it is equal just bent the stopper a little bit back that
the needle will stop at 0MPH-->DONE!
If not You have to take a look at the spring and adjust it/ bent it + or - as long as the readings of bothe speedos are equal.
BUT MAY BE IT IS SOMETHING ELSE, I DONT HAVE IT HERE IN FRONT OF ME.
ALWAYS DIFFICULT TO DO LOONG DICTANCE DIAGNOSTICS
Please take a pictute of the spring but if You don't tried to refill the sillicon oil it should be ok but take a picture first.