Tech article by Honorio Madriaga (madgyver)

Tools needed
  • A flat surface
  • Masking tape
  • 4 feet of string
  • Chalk (for marking)
  • Hydraulic jack with stands (if needed)
Simple five steps
  • Park your rig on a flat surface.
  • Tape one end of the string to the bottom (6 o’clock) of the rear drive shaft. Using a clock method to determine location, bottom would be 6 o’clock, and the top would be 12 o’clock.
  • Mark the rear tire where it touches the ground.
  • Now push your rig forward so that the tire make one full rotation
  • Look at string on the drive shaft.
The string will wrap around the drive shaft and the amount of wrap it has will tell you roughly what gear ratio you got.

If the string is wrapped around the drive shaft 3 ½ times than you have the 3.50 to 1 gear ratio, which came stock behind a 302 V8.
If you got the string wrapped around 4 ¼ times than it will most likely be 4.11 to 1 gear ratio.
If it is wrapped 4 ½ times than it will be 4.56 to 1.
If it is wrapped 4 ¾ times than it will be 4.88 to 1.

If you don’t have room to move the rig, here is another way to do it. Instead of pushing the rig just jack up on rear tire and rotate it one full turn with vehicle in neutral. Count the number of wraps and times it by 2. The total is what you use to figure the gear ratio. Note this procedure will only work with open diffs.