Caliberating the speedometer

Clutch, transmission, rear axle

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bandit76
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Caliberating the speedometer

Post by bandit76 »

I have a 69 F100 with a 360 engine and a C6 tranny. Recently, I had to replace a 3.25 gear ratio rear end chuck with a 3.75. I checked against a GPS speed and I'm running a bit slower that what the truck speedometer is displaying. How can I fix this? Thanks for any help. :fr:
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BobbyFord
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Re: Caliberating the speedometer

Post by BobbyFord »

You need a larger diameter speedo driven gear (more teeth).
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Re: Caliberating the speedometer

Post by Mancar1 »

Bobby knows of what he speaks. :thup:
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Re: Caliberating the speedometer

Post by bandit76 »

Which speedo gear would I need or where could I find the info? I notice LMC has several different ones.
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Re: Caliberating the speedometer

Post by flyboy2610 »

bandit76 wrote:I have a 69 F100 with a 360 engine and a C6 tranny. Recently, I had to replace a 3.25 gear ratio rear end chuck with a 3.75. I checked against a GPS speed and I'm running a bit slower that what the truck speedometer is displaying. How can I fix this? Thanks for any help. :fr:
Well, the way it is, you shouldn't get a speeding ticket! :lol:
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Re: Caliberating the speedometer

Post by BobbyFord »

bandit76 wrote:Which speedo gear would I need or where could I find the info? I notice LMC has several different ones.
I don't have a Ford parts book, but since Ford did not offer a 3.75 option for our trucks you will have to go with your application but with a 3.73 rear gear ratio. If someone here has a parts book they can tell you the correct number of teeth, if not your local Ford dealer can help you.
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Re: Caliberating the speedometer

Post by 70_F100 »

In order to determine the correct gear, we'll need to know what size tires, also.
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Re: Caliberating the speedometer

Post by fordman »

arent you suppose to add one tooth per so many miles per hour it is off?
bandit76
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Re: Caliberating the speedometer

Post by bandit76 »

I found out the rear end chunk came out of a mid-70's ford pickup.

You got that right flyboy speedometer said I was going 60 MPH but reality I was only going 50 MPH.

@ 70_F100 I'm running 235 75 R15 if that helps.
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Re: Caliberating the speedometer

Post by justiz00 »

When I went from a 3.25 to a 3.0 I went 1 tooth lower on the cable side gear. The speedo is off the same as it was with the old gears (incorrect tire size). I found the info here.

http://www.ford-trucks.com/forums/90208 ... tions.html
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