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Discussion Starter · #1 ·
My wifes 67 fastbacks speedo has always been off, I would say 10-12 miles per hour slow. Pulled the driven gear off the cable tonight and it has 17 teeth. C4, so assuming 8 drive teeth, 2.79 rear, 215/65r15 tires. I've tried 4 or 5 different gear calculators and they all say it should be a 17 tooth driven gear. I had an 18 tooth laying around and that made it even slower. Looks like the only smaller gear I can go to is a 16 tooth. Wondering now if the drive gear is wrong, maybe a 7 tooth. Anyone have a tried and true gear calculator?
 

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My wifes 67 fastbacks speedo has always been off, I would say 10-12 miles per hour slow. Pulled the driven gear off the cable tonight and it has 17 teeth. C4, so assuming 8 drive teeth, 2.79 rear, 215/65r15 tires. I've tried 4 or 5 different gear calculators and they all say it should be a 17 tooth driven gear. I had an 18 tooth laying around and that made it even slower. Looks like the only smaller gear I can go to is a 16 tooth. Wondering now if the drive gear is wrong, maybe a 7 tooth. Anyone have a tried and true gear calculator?
Perhaps, this site will help.....https://www.tciauto.com/speedometer-gear-calculator
 

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At what speed is it 10 mph slow? If at 60 you're reading 50. Then you need a 50/60x17 = 14.16...so a 14 tooth driven gear. What drive gear do you have.

16 tooth would be better than what you have now.
 

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Is your odometer slow also? Could be the speedo is out but the odo is right on.
 

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According to the 1975 final edition of the MPC, the drive gear on the 1967 C4 output shaft should have 7 teeth if you have a 6 cylinder engine or a K-code 289, and 8 teeth if you have a non-K-code 8 cylinder engine. The calculator that I developed (Speedometer Gear Calculator | Mustang 428 Cobra Jet Registry) doesn't have an option for a 7-tooth drive gear because those were never used on the transmissions found behind a 428 CJ. You really need to know how many drive gear teeth you're working with if you want the calculators to work, but it's not too hard to try different driven gears (assuming they're available) until you find one that works.
 

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Discussion Starter · #8 ·
At what speed is it 10 mph slow? If at 60 you're reading 50. Then you need a 50/60x17 = 14.16...so a 14 tooth driven gear. What drive gear do you have.

16 tooth would be better than what you have now.
I calculated that I needed a 14 tooth also, but I don't think they make one with less than 16 teeth.
 

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At what speed is it 10 mph slow? If at 60 you're reading 50. Then you need a 50/60x17 = 14.16...so a 14 tooth driven gear. What drive gear do you have.

16 tooth would be better than what you have now.
This is exactly right. You do not need a gear calculator. Your speedo is off by a percentage, change the gear by the same percentage and you will be good to go.

If you cannot go down on the driven gear then you must go up on the drive gear. I am not sure if one must replace the tail shaft on a C4 to change the drive gear or not.
 

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Discussion Starter · #10 ·
This is exactly right. You do not need a gear calculator. Your speedo is off by a percentage, change the gear by the same percentage and you will be good to go.
correct, problem is they don't make a 14 or 15 tooth driven gear, at least I can't find one anywhere...16 is as small as they go.
 

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Did someone in the past change the gears in your differential?
 

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Discussion Starter · #12 ·
Did someone in the past change the gears in your differential?
No, I checked the ratio, its 2.79. I just took the car for a drive and checked my speed with GPS on my phone, 15 miles/hr off at 55 reading on the speedo (70 on GPS). I don't think a driven gear is going to fix that, can only go down 2 teeth. Will see if I can figure out what drive gear is in there, C4 so it should be an 8 tooth.
 

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Will see if I can figure out what drive gear is in there, C4 so it should be an 8 tooth.
Not necessarily. See what I wrote above related to the difference between the C4 output shafts used behind 6 and 8 cylinder engines.
 
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