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Old 10-25-2006, 12:17 AM
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How to determine effective tire diameter

I'm working out transmission speedo gearing for my car. You need the tire circumference (or diameter) to do the math. I measured tire diameter of unloaded tire - but, of course, the tire changes when loaded so the effective tire circumference (or radius @ ground) changes. Is there a rule of thumb on difference between effective tire radius/circumference and unloaded circumference?
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