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speedometer gear selection 66 GT

1018 Views 3 Replies 3 Participants Last post by  rex1965
I have read all the speedo posts but much is contradictory. I have a stock 4 speed toploader. The speedo drive gear is 6 or 7 depending on how it is measured. If you count the starting tooth and the ending tooth it is a 7 tooth. If you count only one end as half of the other end because it is not a complete revolution it is a 6 tooth. I have not seen this addressed. Using the calculator factoring in 25.9 OD tires and the stock 3.00 to 1 rear end 7 teeth gives me a 16.8 gear for the cable and 6 teeth gives me a 14.4 tooth gear for the cable. I could have this backwards but you get the picture. The smallest number of teeth listed in the Mustangsunlimited catalog is 16 anyway. Reading the posts in the disparity between the mph numbers is a nightmare. Please clear this up for me. thanks Bobby
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Thank you so much for your help. 17 teeth it is. I have been turning a wrench for a lot of years but am always happy to learn or at least have an informed second opinion. I am restoring this GT and plan on using 6.95/4 bias ply which the manufacturer list as 25.90 OD which is where I got the other figure to come up with 16.8 based on not knowing for sure how to count the worm spin. I have a stewart warner calibration machine and have previously calibrated my speedo to learn it reads 60 MPH at 58MPH 2 MPH lower than actual speed in other words So with your information I would suspect that 17 teeth is pretty darn close. Great! I wouldn't want to start doing things half way at this point in my life. If I can be of any help let me know. Bobby
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