Speedometer gear choice`
Thread Starter
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5th Wheeling
Joined: Aug 2019
Posts: 29
Likes: 5
Speedometer gear choice`
Hello--
Just want to run this by everyone here before I go and blindly buy an incorrect speedometer gear.
I just swapped my differential from 2.47 to 2.75 on my '79 lincoln continental. However, I didn't grab the speedometer gear from the car I pulled the differential from, and it's now long gone. Tires are 235 75R15. Current driven gear is 16t. Transmission is a C6. Using the calculator on TCI's website, and trying to count the teeth using an endoscope camera, it would appear the drive gear has 9 teeth. If I change the rear ratio from 2.47 to 2.75 on the calculator, the number of driven teeth go up to 17t. Does that seem correct?
Thanks!
Just want to run this by everyone here before I go and blindly buy an incorrect speedometer gear.
I just swapped my differential from 2.47 to 2.75 on my '79 lincoln continental. However, I didn't grab the speedometer gear from the car I pulled the differential from, and it's now long gone. Tires are 235 75R15. Current driven gear is 16t. Transmission is a C6. Using the calculator on TCI's website, and trying to count the teeth using an endoscope camera, it would appear the drive gear has 9 teeth. If I change the rear ratio from 2.47 to 2.75 on the calculator, the number of driven teeth go up to 17t. Does that seem correct?
Thanks!
Lead Driver
Joined: Sep 2003
Posts: 7,018
Likes: 210
From: SoCal
If the only thing you changed is the rear differential gears with the new ratio, the change in driven gear should be:
16 * 2.74/2.47 = 17.67
Of course, you can't have a fraction of a gear tooth, so you will have to settle for 17t, which will make your odometer read about 4% high, or 18t, which will make the odometer 2% low.
16 * 2.74/2.47 = 17.67
Of course, you can't have a fraction of a gear tooth, so you will have to settle for 17t, which will make your odometer read about 4% high, or 18t, which will make the odometer 2% low.
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 787
From: Simi Valley, CA
Just want to run this by everyone here before I go and blindly buy an incorrect speedometer gear.
I just swapped my differential from 2.47 to 2.75 on my '79 lincoln continental. However, I didn't grab the speedometer gear from the car I pulled the differential from, and it's now long gone. Tires are 235 75R15. Current driven gear is 16T Transmission is a C6. Using the calculator on TCI's website, and trying to count the teeth using an endoscope camera, it would appear the drive gear has 9 teeth.
If I change the rear ratio from 2.47 to 2.75 on the calculator, the number of driven teeth go up to 17T. Does that seem correct? No.
Thread Starter
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5th Wheeling
Joined: Aug 2019
Posts: 29
Likes: 5
How are you getting 18? While I don't know the formula, the calculator is giving me 17.26 for the driven teeth number. Also are type 1 and type 3 interchangeable?
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 787
From: Simi Valley, CA
With 2.47 the DRIVEN gear has 16T
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5th Wheeling
Joined: Aug 2019
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Neat! Would you be willing to send me a picture of the catalog, or perhaps which catalog you are using..? The speedometer gear I have is a type 1, not a type 3. Are type 3 gears interchangeable with type 1 gears?
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 787
From: Simi Valley, CA
The D0AZ-17271-B 16T DRIVEN gear is TYPE 1a
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5th Wheeling
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Also, I have a type 1 gear. Are type 3 gears interchangeable with type 1 gears?
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Ford Parts Specialist
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From: Simi Valley, CA
And 1 don't want one, either.
Scope Wizard
Joined: May 2019
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I get 17.78 gear tooth when I calculate using your tire size of 28.88 .. so that would have to round up to 18T... the error appears to be the Tire Diameter ... BUT it is still 18T
I would Trust what Number Dummy Says.
Type 1 and Type 3 gears have a different Pitch IF you look at the diagram Number Dummy Posted ... so I think the answer is NO.
I would Trust what Number Dummy Says.
Type 1 and Type 3 gears have a different Pitch IF you look at the diagram Number Dummy Posted ... so I think the answer is NO.
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5th Wheeling
Joined: Aug 2019
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Quick update--
The 16T gear I had originally was a type 1A, not a type 1. The new gears I bought were type 3A, and are perfectly compatible with the c6.
Despite the advice here, I initially bought the 17T gear. Without knowing fully why I was getting 17.26 and not 18.2 like everyone else, I bought it and tried it out. Instead of being 10mph, it was now 5mph. So, I bought the 18T gear and now its as close as it'll ever be, with it being off by ~1mph.
I am curious as to why the calculators I was using was giving me 17T, and not 18. Maybe I was inputting a number incorrectly?
The 16T gear I had originally was a type 1A, not a type 1. The new gears I bought were type 3A, and are perfectly compatible with the c6.
Despite the advice here, I initially bought the 17T gear. Without knowing fully why I was getting 17.26 and not 18.2 like everyone else, I bought it and tried it out. Instead of being 10mph, it was now 5mph. So, I bought the 18T gear and now its as close as it'll ever be, with it being off by ~1mph.
I am curious as to why the calculators I was using was giving me 17T, and not 18. Maybe I was inputting a number incorrectly?
Thread Starter
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5th Wheeling
Joined: Aug 2019
Posts: 29
Likes: 5
and dont revive a dead thread.
American Powertrain SPUN-10017 American Powertrain Speedometer Gears | Summit Racing