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The speedometer on my '66 GTO reads 14% too high. I'm running larger tires plus a previous owner may have switched rear ends, as the car just turned 98,000 miles. My car had a 3.08 rear end from the factory. When I'm doing 35 mph in town it reads 40 and when I'm doing 65 mph on the highway it reads 74. This indicates to me that I need to increase the number of teeth on the speedometer gear by 14% to get it to read correctly. For example only ... 35 teeth currently X 114% = 40 teeth on the new gear. Does this make sense to you? I'm going to verify the number of teeth currently on the gear, when I change the engine oil this fall and I'll order a new gear at that time.
 
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