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speedometer/tach accuracy ?

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Vehicle Information: Year: 1997; Body Type: Coupe; Total Mileage: 22,500;

I use my 1997 C4S for high speed road rallies where winning times are decided by 100's of a second over 75-100 miles at average target speeds of 110 to 135 mph. Although allowed, I do not use GPS to measure speed. Instead I rely on my tach, speedometer and my navigators stop watch. My speedometer is the traditional stick, not digital. Being aware of the overstatement of the porsche speedometer, i've taken to monitoring speed more by my tach then speedometer especially in the closing miles of these events. I keep a chart of tach REVS to MPH taped to my steering wheel. I've found this very accurate at speeds below 110 mph. However, in a recent event my tach indicated 6200 rpms which translates to a speed of about 165mph in 6th gear. At that point I was actually clocked by radar at 156 mph. My speedometer read above 170. This brings into question the accuracy of the tach readings. Could they be off just as the speedometer. Its also possible that my calculations are wrong. Are there any published formulas that would more accurately compare tach revs to actual speeds? Has anyone measured the accuracy of the tachs? My wheels are stock 18in wheels for the C4S. My tires are Bridgestone SO2s inflated to 38 fr and 45 rear cold. At the conclusion of a race they are generally 5 pounds higher. The tires all have 6,000 miles on them. Thanks, Look forward to your response.

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