It has been awhile since my drag racing days and doing some junk yard scavaging but if the gears where ever changed you may have to
1. jack up the rear of vehicle, put blocks under the front tires
2. put the trans and transfer case in neutral
3. mark a spot on the tire and the driveshaft
4. turn the tire 1 revolution and count how many times the driveshaft turns
3 & 3/4 turns = 3.73
just past 4 times = 4.10
by doing this you can also tell if you have a posi or open rear
posi= both rear tires turn the same direction
open= rear tires spin in opposite directions if rear is an open rear you may have to hold 1 tire still to get driveshaft to turn
it's not perfect but it will get you close
It has been a couple of years since I have done this, if anyone has more to add feel free to do so.
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