what is the factory gear ratio for a 72 4 speed R180 rear end? i have read that it is 3.36-1, but my haynes manual and a couple other place say 3.90-1. i really dont want to tear the cover off if i dont have to

thanks all      sb

It is 3.36     In other parts of the world where Nissan offered the 5 speed transmission in the 240z the ratio was 3.90 for the R180.

 Zhome has all the ratio's listed for reference.

 The ratio can be confirmed by putting the rear of the car on jack stands. Mark the tire and drive shaft with chalk. Either rotate the tire 360` and count the drive shaft revolutions or rotate the drive shaft and count the revolutions it takes to turn the tire 360`

A possibly helpful detail is realizing that .36 is about 1/3, .9 is almost 1, .7 is almost 3/4, and .54 is about 1/2.   Different enough to make it easy to estimate. The first three revolutions are simple, but you don't really need to know the decimals on the final one to determine what you have.  And if one wheel gets ahead of another as you're turning them, just turn the other to catch up, or turn the other back.  As long as they end up at the starting point the math will be right.

If you set yourself up right you can have your face under the propeller shaft while you turn both wheels with your hands.  Use a bright mark and lots of light.