It takes some time to calculate, but this page shows the actual methodology used by carmakers to calculate speedo gear. There is some factored in inaccuracy that will happen no matter what, the point is to minimize it:
http://429mustangcougarinfo.50megs.com/speedo_gears.htmThe page is helpful if you have some sort of modification that would alter the speedometer calibration, in that you can "recalculate" from scratch with your modifications. However, if you are BONE stock: same diameter tires as originally equipped, same rear end gears, and same trans gears, then you can probably get the right speedo gear out of a service manual.
My '79 W72 4-Speed has only one change that would affect what speedo gear to use, and that's slightly wider tires - diameter is just about the same as the factory Polysteel radials, but they're just slightly wider BFG Radial T/A's. After doing the calculations, turns out the factory gray 18 tooth speedo gear that I already use is still the best one.
Don't forget to factor in if you use a speedo gear reducer/adapter - usually installed in the middle of a two-piece "upper" and "lower" speedo cable. I don't think this is likely in the earlier 70's cars, but my '79 came equipped with one. The reducers were usually produced by Stewart Warner.
