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First, there are probably 100 answers to this question and while some of them have a grain of truth to them, I'll bet that true answer is that there is no real reason.
- But -
Most vehicles have 5 lugs, so I supposed symmetry plays a role.
I've heard that using prime numbers has the effect of isolating vibrations. So 5, 7, or 9 are common used when the number of "X" is chosen.
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