gpz clocks on my er5?

hi,i have a 98 er5 and someone has put some early gpz500s clocks on it in a console, the problem is that although the colour coded wires seem to be ok and the tacho does actually work the tacho reads at twice the rpm that it should do any ideas on how to half the pulse to it to read correctly.at the moment it says it ticks over at 2200rpm but its only just over 1000 really.not that it bothers me that much but i have someone who wants to buy it and they are not liking it doing that.