thanks for the previous posts,. but still need more. I have a 81 5 speed in a 71 240z, will the speedo cable work from the 71, can i use a different speedo cable from the 81zx, what is a pinnion gear, and how hard is it to fix.
Use the cable you already have on your 71. When you disconnect the cable from the tranny, the thing it screws into can be removed. At the end of that "thing" (the pinion gear housing) is a little gear called the pinion gear. For your speedometer to give the correct reading, the pinion gear should match the differential ratio you're using. If you're also switching your diff to a 3.9, you already have the correct pinion gear in the 5-speed so you don't need to worry about it.
The gear can be changed to removing the little pin holding it in place.
Yes, the stock 240Z 4-speed should have the correct pinion gear for a 3.36 ratio. The combination of an 81 5-speed and a 3.36 diff will be pretty sluggish unless you have a V-8 in there. You might want to move to a higher ratio diff. I have a 4.11 to go with my 82 5-speed and it hauls ass off the line!