[69L72RS]

I have used this procedure in the past on Zs.

The car has to have the original transmission, or this won't work. You will also need a hand held tach similar to a RC plane prop tach.

1) Pull the speedo gear and determine the tooth count for the set.
2) Hook a drill to the speedo cable and drive the speedo to 50 MPH.
3) Measure the drill RPM.
4) Backfigure to the original gear ratio using using the drill RPM, the speedo gear ratio, and original tire size (diameter) for the car.

It is accurate enough to tell the difference between a 3.73 and a 4.10 rear ratio.

Hope this helps.