There is a fix, using a north/south magnet, but VERY VERY lonmg winded, and seriously better to just get another set.
BUT if you want it's just a case of magnetising the collector drum more to increase reading, or less to decrease reading.
The drum is the drum shaped bit that has the magnet/s spin inside it that are driven from the cable input.
To increase use north end of magnet, decrease use south, with patience and luck you can get it spot on, just a couple of gentle rubs at a time of the magnet will make a difference so little and often for best results.
I think the needle is in the wrong place! Now i had this problem with my Nova when i changed the speedo drive.
No need to be fiddling about with magnets if this trick wont work
borrow yourself a satnav that displays the speed your traveling at, take the front cover off your clocks and drive down the road at 30 (make sure that its VERY straight and VERY empty!!) take the needle off the spindle and place it pointing and 28 MPH (NOT 30mph as if its out too much then you will end up with problems at speed camera time!) then hit the motor way and hit the 70mph (satnav) and it should say something like 75ish on the clocks.