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Our 66 Coupe has had the tranny changed from a 3 speed manual to the C4 automatic. The rearend is a 3:20 gear and the speedo is fast by about 15MPH. What is the correct speedo gear that I need for the transmission?? Or do I need to change both the drive and driven gears?? I have not looked at what the current gears are yet.....
 

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Knowing the rear end ratio is only part of the puzzle. What size tires are you using in the rear. Also where did the gear come from that you are using now? If you say it was the one that was in the 3-speed standard trans that is not good. The gear pitch on a standard transmission and automatic is different. I will however assume that you have the correct gear in the transmission. If your speedometer is reading faster than you are really going then teeth need to be added to the gear. Look Here or here to figure out how many teeth you really need in your speedo gear. fd
 

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sent you an email with an Excel speedo calculator.
 
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