Slim,
Mark is right about marking the driveshaft and the tire, then counting the number of times that the driveshaft turns over for just one turn of the tires. BUT YOU HAVE TO MARK BOTH tires and make sure both tires turn only one turn, posi or non-posi rear. This is more difficult to do if it is a non-posi because only one tire will try to turn, it's good to have a helper. Otherwise you will get an incorrect ratio.
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