125-200cc SPEEDOS

My bike has a speedo a bit like this
http://www.ebay.co.uk/itm/NEW-MINI-...85?pt=UK_Motorcycle_Parts&hash=item5b010d769d

but it's very inaccurate, even with a new cable, it thinks I'm doing between 40 and 45 mph when I know I'm doing 30...

so I just bought a bicycle one, digital, with a wire and two magnets I think, I'm going to mount it on the handlebar then the other magnet on the left fork, as it says for mountain bikes with adjustable wheel sizes I'm hopeful it will be enough for a 21 inch husky wheel! Has anyone else tried to do this? What was the accuracy like?

The bicycle style speedo came stock with my Beta trials bike. I tried to make it work and determined I was better off with mounting a handheld gps.