I suggest that you try one of these for the most accurate and simplest to install form of speed, tach and other functions intrumentation.
I have one on my KLR and it's great. Not expensive and very rugged. Small in size but easy to read.
The tach and speed are simple to hook up. Mine even has a water temp display. And you can program two separate shift lights.
Computers: Digital Speedometers and Tachometers at Trail Tech Home
It will give you top speed, clock, trip odo, total odometer and stop watch to boot!
Sure beats trying to make a cable drive work at the proper ratio. This one is all programmable for any size tire or engine.