As noted in previous posts, start with the fit of cables, although those can be made special if needed. Next is the combination of wheel size and drive gear ratio for the speedo, and the tach drive ratio in the engine. As reed says, it's all trial and error. However, since the tachs and speedos on early Japanese bikes are all similar mechanical devices, they may turn out to be pretty close, especially if you're just mostly interested in the look of these particular items. You can do mental corrections as you ride even if they aren't accurate. In the case of the speedo, both the original on my GTR and a new replacement from Richard are 15% - 20% optimistic, so all the calculations in the world may not mean much. I really don't know how accurate the tach is, but the engine response curve and the red line numbers seem to correspond pretty closely with what I would expect.
