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In a posi you have to overcome the friction in the discs. When you jack up the rear and spin it, it is like a locked spool so you get the proper number of revolutions. In an open diff the spider gears just spin and the other tire goes backwards. You have to stop the other tire from spinning like other have said.I have found the easiest and most reliable way is to pull the cover and rotate the ring gear until you can read the stamped ratio on the side of the ring gear!!! Plus...when was the last time you changed oil in the diff?