Here's one way..Jack the rear and put it on stands. Make a mark on the driveshaft, then turn the tire one complete revolution, counting the number of times the driveshaft turns. If the driveshaft turns roughly 3 times, it's a 3.08 gear. 3 1/2 times, it's a 3.42; 3 3/4 times it's a 3.73; 4 times it's a 4:10..
Edit: 4 1/2 times it's a 4:56 if it has very low gears.
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