Speedo Cable Driven gear

[drew8mc]

Hello all,
My speedo has been running super fast. I would say when I'm doing 35, it's saying I'm doing 50.

I pulled my speedo cable out of the tranny (200 4r) and it looks like I have an orange drive gear (11 tooth from what I've researched) and a 30 tooth driven gear. When I use the online calculator, even though I don't know my rear end ratio, the driven gear seems to vary only from 28 to 32 teeth if I estimate the rear end from a 3.55 to a 4.10. Doesn't that seem like my 30 tooth gear should be much closer than it is? I read somewhere that adding a tooth to the driven gear would decrease the speedometer by 2.5% which seems like I would need about 42 teeth to get it in range.

How do I confirm my rear end ratio without pulling the cover?

[LockDoc]

Originally Posted by drew8mc

Hello all,
My speedo has been running super fast. I would say when I'm doing 35, it's saying I'm doing 50.

I pulled my speedo cable out of the tranny (200 4r) and it looks like I have an orange drive gear (11 tooth from what I've researched) and a 30 tooth driven gear. When I use the online calculator, even though I don't know my rear end ratio, the driven gear seems to vary only from 28 to 32 teeth if I estimate the rear end from a 3.55 to a 4.10. Doesn't that seem like my 30 tooth gear should be much closer than it is? I read somewhere that adding a tooth to the driven gear would decrease the speedometer by 2.5% which seems like I would need about 42 teeth to get it in range.

How do I confirm my rear end ratio without pulling the cover?

You might check and see if their is a code stamped on the rear end. Check posts #8 and #19 in this thread for possible code letter locations and use the charts in the thread to look it up....

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

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[drew8mc]

Thanks, I looked quickly but didn't find it. It's absolutely covered in oil. It def has a bad seal where the driveshaft connects. It has spayed oil all over the rear end. I'll have to rebuild the it anyway, just wasn't planning on it right away.

I think I figured out why it is so off. The speedo cable was actually attached to another housing that turned it 90 degrees which was being driven by the 30 tooth gear. I'm betting that is some sort of adapter for a previous setup and that's why it is throwing it so off.

I tried to check my ratio by jacking it up and moving the tire to see how many driveshaft rotations there were, but the result is strange. It only took 1.75 turns to rotate the passenger side tire one revolution. The other side was locked solid. So no posi for me for sure, but did I do that wrong? That's no where close to a real ratio.

[LockDoc]

Originally Posted by drew8mc

Thanks, I looked quickly but didn't find it. It's absolutely covered in oil. It def has a bad seal where the driveshaft connects. It has spayed oil all over the rear end. I'll have to rebuild the it anyway, just wasn't planning on it right away.

I think I figured out why it is so off. The speedo cable was actually attached to another housing that turned it 90 degrees which was being driven by the 30 tooth gear. I'm betting that is some sort of adapter for a previous setup and that's why it is throwing it so off.

I tried to check my ratio by jacking it up and moving the tire to see how many driveshaft rotations there were, but the result is strange. It only took 1.75 turns to rotate the passenger side tire one revolution. The other side was locked solid. So no posi for me for sure, but did I do that wrong? That's no where close to a real ratio.

With an open rear end you have to turn the tire 2 revolutions.

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