The Ultimate Pontiac GTO Picture Site By Sean Mattingly.
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If you have an open (non-posi) rear, then you must multiply the number of driveshaft turns by 2 to get the gearing (i.e. 1.5 turns would be a 3.0 rear [1.5 * 2 = 3.0], or a 3.08 more likely). The reason for multiplying by 2 is that in an open differential, the planetary gears cause the driveshaft to rotate half as fast as in a posi if only one tire is being rotated. Therefore, you must multiply the drive-shaft turns by 2 to get an accurate ratio, otherwise you must rotate one tire then the other (and not multiply). When one tire is not moving, the planetary gears are "walking" around the differential housing in the opposite direction causing the drive-shaft to rotate slower than if the diff were "locked".

Old GTOs came with tires with different codes. I converted these codes to the current code types. Although some of these codes are not available as I converted them, it's important that the circumference is calculated correctly. This is true for both calculating Top Speed back when the car was sold as well as keeping an accurate speedometer with today's tires. The reference I used for tire size conversions can be found at tirerack.com.

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