I found that the tach could be adjusted with the speedo still attached by bending the little adjuster out a little. This way, if your speedometer is correct, you can calculate what the RPM should be at any speed and adjust accordingly. I made a little excel calculator to help with this. This calculator is to be used with your transmission in its 1:1 gear. You input tire size and rear end gear ratio. It goes from RPM to speed and the other way around. Let me know if it's useful or if I could make it more helpful.

https://docs.google.com/spreadsheet/...Ew1b1dhekkxM2c