elp..Speedometer Problems

HELP !! I 'm having problems installing replacement speedos on my 1981 KZ 440 LTD. The problem is that everytime I put a replacement Speedo and cable on, and I try the bike out, the needle goes crazy and frequently breaks.

I've attempted this now with 2 different speedos and my bike guru does not have any idea whats causing this issue.
Any suggestions ? I welcome all ideas and suggestions!!
Thanx
Motorcycle Michael
Chino Valley, Az.

Wrong ratio on the new speedo?

Maybe the speedo INNER cable is too long? Instead of the magnetic eddy currents from the rotating magnet driving the needle, the rotating cable INNER is pushing the magnet against the "cup" which carries the needle causing it to rotate at a much higher rate than it should be...? - just a suggestion, the gap between the magnet and the "cup" is not very much and it doesn't take much for these two items to touch, thus causing the needle to be driven directly by the cable.

What speedos have been tried? If non-stock, KZs use the 2240-type of speedometers used on most if not all Japanese motorcycles.