need help with speedometer / gearing
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Senior User
Joined: Jul 2013
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From: Central Florida
need help with speedometer / gearing
1977 F-150 460 np435 35inch tires.
So im tackling why my speedometer doesn't read what MPH im going. I hooked my cordless to my Speedo cable and it works just fine. The gear itself (yellow) looks in fair condition but I don't know what it looks like new. So my next step is to get the gearing. No tag. So I count pinion rotation... 1.75 turns to one full tire rotation. Seems a bit low to me... Considering the truck runs out of gear (leaving in second) really quick and gets to going fairly fast with not a lot petal needed.... I was thinking it was more like 3.73 (which it is not)
So a long list of questions.
Is my Speedo gear bad?
What is my exact rear end gear?
If my Speedo gear is bad what gear do I need with 35 inch tall tires and whatever gear I have?
To make thing worse I was planning on gearing down the truck to make second gear last longer and a longer 4th. That is no longer an option cause that's way to thin a gear to support any real power.
Thanks in advance.
So im tackling why my speedometer doesn't read what MPH im going. I hooked my cordless to my Speedo cable and it works just fine. The gear itself (yellow) looks in fair condition but I don't know what it looks like new. So my next step is to get the gearing. No tag. So I count pinion rotation... 1.75 turns to one full tire rotation. Seems a bit low to me... Considering the truck runs out of gear (leaving in second) really quick and gets to going fairly fast with not a lot petal needed.... I was thinking it was more like 3.73 (which it is not)
So a long list of questions.
Is my Speedo gear bad?
What is my exact rear end gear?
If my Speedo gear is bad what gear do I need with 35 inch tall tires and whatever gear I have?
To make thing worse I was planning on gearing down the truck to make second gear last longer and a longer 4th. That is no longer an option cause that's way to thin a gear to support any real power.
Thanks in advance.
Post Fiend
Joined: Jan 2006
Posts: 5,037
Likes: 9
From: Kasson, Minnesota
Your NP435 probably has an 8 tooth Drive Gear, so assuming your rear end is 3.73 and 35 inch tires, it calculates for a 17 tooth driven gear (white). There probably isn't anything wrong with the gears, just need a different tooth gear for the speedometer.
Postmaster
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Senior User
Joined: Jul 2013
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From: Central Florida
Woah, slow down kids! My rear end gear is NOT 3.73, I was saying I thought it was much higher than what it is.
According to counting the pinion the gear ratio is 1.75, which isn't even produced. The lowest I can find was 2.70.
The Speedo doesn't work at all while driving. I have only got my truck not much faster than 60mph (im guessing) and it still doesn't budge.
So I can't figure out what my rear gear actually is.
I don't know where to start
According to counting the pinion the gear ratio is 1.75, which isn't even produced. The lowest I can find was 2.70.
The Speedo doesn't work at all while driving. I have only got my truck not much faster than 60mph (im guessing) and it still doesn't budge.
So I can't figure out what my rear gear actually is.
I don't know where to start
Postmaster
Joined: Sep 2013
Posts: 2,595
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From: Pasadena, Ca
So, your gauge doesnt register an mph, the cable is verified working.... So you are troubleshooting the gauge?
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Post Fiend
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Post Fiend
Joined: Jan 2006
Posts: 5,037
Likes: 9
From: Kasson, Minnesota
Oh that's right. Now I'm guessing that it's the drive gear in the transmission.
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MikeGV
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Sep 26, 2005 09:30 PM