Originally Posted by Tango
The ratio of any axle can be determined by doing the following:
1. Block front tires and jack up driver side rear with the vehicle in neutral
2. Put matching marks on the ground and driver side tire
3. Put a mark on the drive shaft
4. Rotate the driver side tire one full revolution while counting the number of turns on the driveshaft
That is your ratio
I hate to disagree with you, but I do not want someone using this method and assuming they have the wrong gear ratio.
If you only jack up one wheel, you will be driving the wheel through the spider gears, this is a second set of gears in the diff that will give a faults ratio.
See this web site:How does a Differential work ? ~ Learn Engineering
As you can see from this page, the Side gears are blue and the spiders are yellow. When you rotate the drive shaft it turn the carrier, this drives the spider gears, but if one side gear is held still(I.E. being on the ground) the other wheel(Side gear) is driven by the spiders rotating around the fixed side gear and rotating the free side gear(wheel in the air). Often times at a rate of 2:1, thus giving you a wrong ratio.
So to use your method, you need to have both wheels off the ground so that the only gears involved are the pinion and the ring.
Just want to make it clear for all in the future who may want to use this method.