FZ 600 S2

I brought my wife's 2007 FZ 600 S2 Fazer to the UK from South Africa. This model is sold with a KPH only speedometer in South Africa. In order to get it registered in the UK, I had to fit a UK spec speedometer, which I duly did. The UK spec speedometer can show both MPH and KPH, probably to facilitate use in Europe. However, it now shows wild speed readings.50 mph shows as 82 MPH and 118 KPH!!!!!!!!!!!!! It has been suggested that I need to change the speed sensor as well. Any views on this?

Regards

Delvin