Look at the rear axle code stamped on the housing or jack up the rear and rotate the tire 1 revolution and count how many times the driveshaft turns. Put a mark on the tire and driveshaft so you know when to stop. Although primative it's close. You can always count your gear teeth in the rear.
What's the tranny? Tell us what you rev at 60 and 70mph in fourth or high gear in an auto.
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.
The best way is to get the axle code off the rear end. This is stamped on the passanger axle tube facing front about 1/2 way between the center and end. There will be some letter combos, This gives you the ratio and other good info. I have codes from 69 up at home if you find them.
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