Speedometer gears
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Mountain Pass
Joined: Mar 2014
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Speedometer gears
Going from a np435 4 speed to a c6 auto in a 76 f350. Will the speedometer drive gear on the cable from the manual work in the auto and be as accurate as it was? I looked and can’t find the info I’m looking for. Thanks
Fleet Mechanic
Joined: Dec 2007
Posts: 1,502
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From: Wildwood Pa
You'll need to count the teeth on the drive gear in the transmissions. If the tooth count is the same between the two transmissions, & you use the same tooth count driven gear, the speedo will read the same.
There is no way to know for sure what drive gear you have for sure without counting the teeth.
There is no way to know for sure what drive gear you have for sure without counting the teeth.
More Turbo
Joined: Jun 2020
Posts: 638
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From: Montana
You can go to this site and use it to calculate what you what driven gear you need based on the tire size, rear end ratio, and drive gear. You still need to know the drive gear tooth count though... I'd think a flashlight and marking device may allow you to put the tranny in neutral, spin the driveshaft and count the teeth on the drive gear through the hole the speedo cable attaches to.
I think the simplest option is probably gonna be to drive it, the faster the better for accuracy, and compare the speedometer speed vs GPS speed, calculate the difference, count the teeth on the driven gear attached to the cable, buy a new one that has the correct number of teeth, realize you went the wrong direction in your teeth, buy a new gear changing the right amount of teeth the other way and have it work right.
I think the simplest option is probably gonna be to drive it, the faster the better for accuracy, and compare the speedometer speed vs GPS speed, calculate the difference, count the teeth on the driven gear attached to the cable, buy a new one that has the correct number of teeth, realize you went the wrong direction in your teeth, buy a new gear changing the right amount of teeth the other way and have it work right.
Logistics Pro
Joined: Jul 2005
Posts: 4,217
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And you can try getting an assortment of driven gears from the junkyard, and see if one of them is correct.
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Fleet Mechanic
Joined: Dec 2007
Posts: 1,502
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From: Wildwood Pa
If both the manual & auto transmission came out of trucks with similar rear end ratios, you should be fine. The drive gear should be the same tooth count. You mentioned 3.73 & 4.10, those should use the same drive gear. But, these trucks are 40+ years old, & a lot has been swapped over the years, so it is not a sure thing. You can swap the driven gear to correct a speedo that is off, but sometimes the speedo is too far off for that to work. In that case, you have to go into the transmission & change the drive gear.
Hopefully luck is on your side & it all works as is for you.
Hopefully luck is on your side & it all works as is for you.
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Mountain Pass
Joined: Mar 2014
Posts: 143
Likes: 4
Well I finally took it for a spin, at 60 the speedometer is in the 80 85 range. So tomorrow I’ll swap in the gear that was with the c6 and see what that gets me. If it’s close enough I’ll leave it and just figure out what it’s telling me. I’ll make it more accurate after I upgrade it to radial tires. Thanks!!
Lead Driver
Joined: Jul 2017
Posts: 9,545
Likes: 1,156
From: San Jose, CA
For the speedo drive gear, you can see it through the hole in the transfer case intermediate housing, but I imagine it's pretty hard to determine through that narrow window. It's a wire spiral splined gear and I think the two possibilities were either a 6-tooth or a 7-tooth gear. Not sure if Ford made more than that, but those are the ones that come up most often in other discussions.
Worth taking a peek in there anyway. Remove the cable and use a bright light and some patience. Hopefully you can get your head up in there!
Paul
Lead Driver
Joined: Jul 2017
Posts: 9,545
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From: San Jose, CA
I think that represents about a 25% speed difference, and the max range of the smallest to largest gears is right about there.
At least as far as our availability, it's only 16t to 21t range (https://www.wildhorses4x4.com/produc..._Transfer_Case) so hopefully you have a 16 on there now and the one from the C6 is 21t.
I believe the gears are the same for your 205 because I seem to remember changing them in my '79 and they were the same as my '71 Bronco.
Good luck.
Paul
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Mountain Pass
Joined: Mar 2014
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The one I pulled out was a 16 tooth, I put in a 20 tooth. It’s a lot closer now, about 5 off now. I’ll have to buy a 21 tooth and give that a try. At least it’s close enough to use so I don’t have to keep my phone in the windshield haha
More Turbo
Joined: Jun 2020
Posts: 638
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From: Montana
There's an 8 tooth also because I have one on my workbench and it's a NOS Ford part.
Lead Driver
Joined: Jul 2017
Posts: 9,545
Likes: 1,156
From: San Jose, CA
Good to know! I wonder what applications got them? For the Early Broncos it was probably just the other two simply because the gear ratio choices were exactly two, and the tire size choices were exactly two also if I remember. Not much call for another drive gear I would not imagine.
But it might help those that make major changes to both gearing and tire size.
Paul
But it might help those that make major changes to both gearing and tire size.
Paul
Lead Driver
Joined: Jul 2017
Posts: 9,545
Likes: 1,156
From: San Jose, CA
Nice! Glad when a plan comes together like that. And glad too that this is a relatively inexpensive part (if you have to buy one) and a relatively easy part to change out.
So you don't have to pound your table in frustration when you find out you have to do it again!
Not that we'd ever have to do that of course...
Paul
So you don't have to pound your table in frustration when you find out you have to do it again!
Not that we'd ever have to do that of course...
Paul