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Speedo gear questions

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Here's a question I posted on the "old" board yesterday. Thought I'd try here. I did get a good answer from "atf" based on the Corcoran & Davis Resto Guide, which says I should go with the C0DD-17271-A; type 3a, nylon, color wine, 16 teeth. I think that's right too, but I thought I'd try again here.

While working up the nerve to tackle my heater problem, I decided to replace the bushings in my shifter mechanism. All went well with that and now I have a very tight shifter for my C4! While in the vicinity, I decided to see how many teeth are in the gears of my speedo cable at the transmission. My speedo reads about 10 mph low at 50 mph. It's easy to count the teeth on the driven gear (mine has 19), but I couldn't find any way to actually see the drive gear. I could feel it, however, so I used my fingernail and counted the bumps I felt. It was either 7 or 8, but I couldn't be sure which. This is a C4 with a I6/200 on a 67 Coupe. How do you guys determine this without taking the tranny off?

Using the formula in the Mustangs Unlimited catalog, 26" tire diameter, and the 2.83 rear end ratio (from my VIN number), I get a driven gear teeth count of 17.56 with 8 drive teeth and 15.37 with 7 drive teeth. Does this make sense? Which driven gear do I really need? Since neither number is close to the 19 teeth I actually have, I'm hopeful that this is what's causing my speedo error. Right?


Randy (Learning as I go)
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6,700 Posts
You've got a 19 tooth gear and the speedo reads high. If you get a gear with less teeth, it will speed up speedo cable's rotations, causing the speedo to read higher for a given speed. Therefore go get a 17 and 18 tooth gear. Install one and perform some speed/milage checks. Either of those two will work out the best. What I did was drive along a freeway and record milage markers versus the odometer in the car. did a little math, and found out which on was closest (it turned out that one was 2.5% low and the other was 2.5% high, so I chose the high one, so help save my butt on tickets).

Steve Leslie, 65 coupe in restoration. 302, toploader, A/C, disc brakes, bench seat
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