bicycle speedometer for a mtoorcycle ???

Hey guys, so my zs250gs doesn't have gages and I came across a video on youtube of a bike with a bicycle computer adapted on a motorcycle. Excited I went to wallmart bought a 20$ BELL computer and put it on my bike. Well aparently it doesn't work quite well. Works fine up until about 15 mph, then I guess that the wheel spins so fast that the sensor does not recognize the pasage of the magnet that I place on the spokes.
I'm wondering if getting a stronger magnet from radio shak would solve the problem ???