How do you check gear ratio by spinning tire and counting pinion rotations?

How do you check gear ratio by spinning tire and counting pinion rotations?
Jack up the vehicle on one side. mark the drive shaft with a piece of chalk or tape a piece of string to the drive shaft, at a specific location, say at the point closest to the ground. Also mark the tire as well so that you can determine exactly how many revolutions you turn the wheel. If you don't use the string method, and simply mark the drive shaft, you'll need someone else to count how many rotations the drive shaft makes while you count the number of times the tire is turned.
If you just need a good estimate, turn the wheel exactly 2 times. Write down how many revolutions and partial revolutions the drive shaft turned, i.e. 4.3 or 3.7 or 4.5, etc. This should give you a good enough estimate to determine which gear ratio you have once you consult a dealership as to which possible gear ratios exist for that model vehicle. If you don't want to call a dealership, etc, you can get the EXACT ratio simply by turning the wheel 20 times (instead of 2), and then dividing the drive shaft revolutions by 10.

Why? Because normally one would turn the tire 10 times, and then divide the result by 10, but if the differential is an "open differential", meaning each tire can turn independently of the other, then the internal gears compensate when only one tire turns, and it takes twice as many tire revolutions to get the ratio, hence 20 turns, even though you're dividing the drive shaft revolutions by 10.

For example, let's say you turn the wheel 20 times and the drive shaft turns exactly 43 times. Then you have a 4.3 gear ratio because 43 / 10 is 4.3.

Hope this answered your question, and good luck!