That will work with a sure-grip only. With an "open" diff there is a 2:1 ratio. If you want an accurate ratio estimate for either type of diff you need way more than one tire revolution. Here is a good example: Mark a reference line on both the driveshaft and on the tire. With a sure-grip, rotate the tire exactly 4 revolutions. Have a helper count the revolutions of the driveshaft. Let's say the shaft rotated "about 14" times. Dividing 14 by 4 we get 3.50, so its probably a 3.55 gear. The more revs you make, the more accurate the final number.
The same scenario for an "open" diff would be: Rotate the tire exactly 8 times, while your helper counts the driveshaft revs. Same as above, the shaft rotated around 14 times. To figure gear ratio divide 14 by 8, and then multiply by 2. Same answer, somewhere around 3.50.
