Speedo calibration
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Tuned
Joined: Dec 2016
Posts: 296
Likes: 21
From: Ohio
Speedo calibration
I have a 78F150 with 3.55 gears. It has the NP435 and the NP205. Due to 35" tires the speedo is off. Using a GPS when the speedo reads 60 I am actually doing 70. Can anyone tell me how many teeth I need to lose or gain on the cable gear to correct this.
Grumpy goat
Joined: May 2017
Posts: 3,058
Likes: 594
You need more teeth on the new gear. Pull yours, they're color coded. You may also do what I did. I'm running 32" tires, switched speedo gears (16 I think, but not sure), and I replaced the 85 mph speedometer with an earlier model 100 mph speedometer. It's calibrated perfectly. Not that my combination will fix yours, just letting you know there's more you can do besides changing the gear.
More Turbo
Joined: Jun 2020
Posts: 643
Likes: 388
From: Montana
Here's a site that can help with the math. Ford Speedometer Calculator You can just ignore the PSOM part. Buuuuuut, assuming a bit here, if your truck works like my '79 with a NP205/3.55gears/33"tires you won't be able to quite get there without changing the drive gear or something else also. I changed from a 17 tooth to a 16 tooth and I'm within about 1 mph error at 70mph. I don't know of any 15 tooth driven gears and that's what I think you'd need for 35s. If you want a new 8 tooth drive gear, the normal old parts sources like ebay are required to get them, but you have to take your transfer case apart to swap the drive gear.
Fleet Mechanic
Joined: Dec 2007
Posts: 1,504
Likes: 339
From: Wildwood Pa
I disagree with ranger, to make a speedo read faster you need fewer teeth on the driven gear, not more.
Pull your driven gear, tell us how many teeth are on it. You need to lose roughly 15% of the teeth to get it more accurate. That may or may not be possible, depends on how many teeth are on your current gear.
All speedo heads, either 85 or 100 MPH, are designed to read the same at a given input. Spin the input at 1000 RPM, they should read 60 MPH. But, they are mass produced, & 40+ years old & there is some variation. Out of a hundred speedos tested, maybe 5 have read off more than 3-4 MPH off from where they should. Always read slow, I've never had on read way fast. So, if you want to try a bunch of speedo heads, you can get a correction that way as well, as ranger mentions. Be aware that this will not correct the odometer error.
If a gear change gets you close, & you want to try a different speedo head to get you the rest of the way, I can test what I have & may have something that will get you where you want to be.
Pull your driven gear, tell us how many teeth are on it. You need to lose roughly 15% of the teeth to get it more accurate. That may or may not be possible, depends on how many teeth are on your current gear.
All speedo heads, either 85 or 100 MPH, are designed to read the same at a given input. Spin the input at 1000 RPM, they should read 60 MPH. But, they are mass produced, & 40+ years old & there is some variation. Out of a hundred speedos tested, maybe 5 have read off more than 3-4 MPH off from where they should. Always read slow, I've never had on read way fast. So, if you want to try a bunch of speedo heads, you can get a correction that way as well, as ranger mentions. Be aware that this will not correct the odometer error.
If a gear change gets you close, & you want to try a different speedo head to get you the rest of the way, I can test what I have & may have something that will get you where you want to be.
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Tuned
Joined: Dec 2016
Posts: 296
Likes: 21
From: Ohio
Going by the calculator up above it does look like I will need a 15 tooth gear. Wanting to say it has a 17 in it but my memory is foggy. I did find an 8 tooth gear on fleabay. Too bad I hady 205 apart last year.
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Cargo Master
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speaking of cables ,mine is out of the truck, and seams sized .Anyway to free them up ? or new cable
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Grumpy goat
Joined: May 2017
Posts: 3,058
Likes: 594
I disagree with ranger, to make a speedo read faster you need fewer teeth on the driven gear, not more.
Pull your driven gear, tell us how many teeth are on it. You need to lose roughly 15% of the teeth to get it more accurate. That may or may not be possible, depends on how many teeth are on your current gear.
All speedo heads, either 85 or 100 MPH, are designed to read the same at a given input. Spin the input at 1000 RPM, they should read 60 MPH. But, they are mass produced, & 40+ years old & there is some variation. Out of a hundred speedos tested, maybe 5 have read off more than 3-4 MPH off from where they should. Always read slow, I've never had on read way fast. So, if you want to try a bunch of speedo heads, you can get a correction that way as well, as ranger mentions. Be aware that this will not correct the odometer error.
If a gear change gets you close, & you want to try a different speedo head to get you the rest of the way, I can test what I have & may have something that will get you where you want to be.
Pull your driven gear, tell us how many teeth are on it. You need to lose roughly 15% of the teeth to get it more accurate. That may or may not be possible, depends on how many teeth are on your current gear.
All speedo heads, either 85 or 100 MPH, are designed to read the same at a given input. Spin the input at 1000 RPM, they should read 60 MPH. But, they are mass produced, & 40+ years old & there is some variation. Out of a hundred speedos tested, maybe 5 have read off more than 3-4 MPH off from where they should. Always read slow, I've never had on read way fast. So, if you want to try a bunch of speedo heads, you can get a correction that way as well, as ranger mentions. Be aware that this will not correct the odometer error.
If a gear change gets you close, & you want to try a different speedo head to get you the rest of the way, I can test what I have & may have something that will get you where you want to be.
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