Speedometer gearing... What's in the rear? - Mustang Evolution

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Old 10-05-2014, 09:32 PM   #1
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Speedometer gearing... What's in the rear?

It's a 1993 cobra with 245/45/17 tires

I just pulled the tranny to put in a new clutch and the speedo gear is worn out. It's got 23 teeth and a 6 tooth drive gear from what I can see in the hole where the speedo gear goes into the trans (I haven't opened the tranny up yet)

I don't know if the rear end has new gearing but this setup doesn't make sense to any rear gearing with any of the online calculators.

What am I missing or is this car just stock and everything is fine with the number of teeth. I need to get a new gear in it.

Before I just throw another 23 tooth speedo gear in, I was going to try and figure out what rear gears were in it based on the setup seen so far but I can't do the math right...


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Old 10-05-2014, 09:52 PM   #2
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I just ran the #s and it came to 4.88 in the rear... Is this plausible? Or am I doing this wrong


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Old 10-06-2014, 06:58 AM   #3
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What method did you use to get the 4.88 gears?
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Old 10-06-2014, 07:30 AM   #4
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What method did you use to get the 4.88 gears?
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Old 10-06-2014, 07:54 AM   #5
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Speedometer gearing... What's in the rear?

Turn the rear tire one complete rotation and count how many times the differential rotates. This will give you your rear gear ratio. (As long as those tires are close to stock diameter. Otherwise, you are just getting the effective ratio. Might make more sense to pull the wheel off first and then turn the rotor instead.)


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Old 10-06-2014, 12:46 PM   #6
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Turn the rear tire one complete rotation and count how many times the differential rotates.
Wow, why I didn't think of this yesterday... hahaha. Thanks.

I used some calculator for the speedometer gearing I found online. And most likely it is wrong...
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Old 10-06-2014, 01:11 PM   #7
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...As long as those tires are close to stock diameter...

Tire diameter makes no difference unless you are trying to convert engine rpm to speed. If determining differential ratio, make sure both tires are off the ground and rotating the same, otherwise you will get a screwed up ratio.
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Old 10-06-2014, 01:58 PM   #8
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Tire diameter makes no difference unless you are trying to convert engine rpm to speed. If determining differential ratio, make sure both tires are off the ground and rotating the same, otherwise you will get a screwed up ratio.
The car is up in the air so no worries. I'll check tonight and post back see how terribly off it was. I also ordered a replacement 23 tooth gear. Will post back when I turn the tires.
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Old 10-06-2014, 02:27 PM   #9
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Tire diameter makes no difference unless you are trying to convert engine rpm to speed. If determining differential ratio, make sure both tires are off the ground and rotating the same, otherwise you will get a screwed up ratio.

That would make sense. Posted right out of bed this morning and my brain was not up to full speed.


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Old 10-06-2014, 05:30 PM   #10
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The car is up in the air so no worries. I'll check tonight and post back see how terribly off it was. I also ordered a replacement 23 tooth gear. Will post back when I turn the tires.
As long as you have the car in the why not use this method to find your rear gear ratio.. doesn't get any easier than this.

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Old 10-07-2014, 01:57 AM   #11
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Tire diameter makes no difference unless you are trying to convert engine rpm to speed. If determining differential ratio, make sure both tires are off the ground and rotating the same, otherwise you will get a screwed up ratio.
only one tire is rotating when I spin each side... I also have my transmission out as I'm replacing my clutch and flywheel + other parts

Pinion rotated about 17.7 times for 10 tire rotations on the drivers side with the other stationary...

so 17.7/(10/2) = 3.55 gears? gonna try this again...

20 tire turns yielded 35.5 rotations so 3.55 gears are in the car. I thought it was a 6 tooth drive gear but looks like now 8
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Old 10-07-2014, 02:39 AM   #12
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I'm using this to get the calculation of what I should do to correct it:

Mustang Speedometer Gear Calculator
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Old 10-07-2014, 02:53 AM   #13
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The speedo drive gear in the transmission is like an off white cream color. I found this on another thread: "the 8 tooth gears are sometines yellow-ish, cream/off-white, or sometimes light green..."

how can I be sure it's 8?

edit: with an 8 and that calculator above it shows I need a 22.3 tooth speedo gear...
edit 2: I guess 23 is as close as I can get... however the gear on the driven is shaved down (possibly it didn't mesh well?)
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Old 10-07-2014, 07:27 AM   #14
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only one tire is rotating when I spin each side... I also have my transmission out as I'm replacing my clutch and flywheel + other parts

Pinion rotated about 17.7 times for 10 tire rotations on the drivers side with the other stationary...

so 17.7/(10/2) = 3.55 gears? gonna try this again...

20 tire turns yielded 35.5 rotations so 3.55 gears are in the car. I thought it was a 6 tooth drive gear but looks like now 8

Only one tire turns? I was always under the understanding that if you spin one tire and the other spins the same direction, you have Posi. If the other tire spins the opposite direction you have an open rear end. Never heard of the other tire not spinning at all. Maybe someone else can jump in here. This is the first Ford I have ever owned. Maybe they are different? I know this wasn't what your question is about, but I think it would be good to know.


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Old 10-07-2014, 08:07 AM   #15
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Only one tire turns? I was always under the understanding that if you spin one tire and the other spins the same direction, you have Posi. If the other tire spins the opposite direction you have an open rear end. Never heard of the other tire not spinning at all. Maybe someone else can jump in here. This is the first Ford I have ever owned. Maybe they are different? I know this wasn't what your question is about, but I think it would be good to know.
The brake is holding the other wheel which will cause the driveshaft to turn twice as much. You were right about the positrac differential meaning that this one does not have one.
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Old 10-08-2014, 01:16 PM   #16
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Don't use the 23 tooth gear, its a chrysler gear not a Ford gear and will get eaten up again.

Steps to check gear ratio. I have 0 clue why you are spinning your tires like 10 times btw...

-Jack the rear of the car up
-Mark the driveshaft with a piece of chalk
-Take your tire and by hand spin int around in exactly 1 complete rotatin
-As you are spinning it, watch the Driveshaft, count how many times it has turned.

3 and 3/4 turns=3.73s, 4 turns=4.10s and 3 and 1/2 turns=3.55s etc...

Also if the 23 tooth fail gear was in there it almost 100% has the stock 8 tooth drive gear which will not work with anything above 3.27s and correct Ford speedo gears. To correct for anything above 3.27 you need to pull the tailshaft and swap the worm gear to a 7 or preferably 6 tooth. 7 tooth allows you to calibrate for up to 3.73s with the red 21 tooth gear. 6 tooth you can go up to 4.56 I believe.
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Old 10-08-2014, 01:36 PM   #17
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With 3.55s on a 245/45/17 I would need:

On a 6 tooth I would need a 16.7 tooth speedo.

On a 7 tooth 19.5 tooth.

On a 8 tooth a 22.3 tooth

Where are you getting 6 tooth is for over a 3.27 rear?


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Old 10-08-2014, 01:41 PM   #18
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17 - 16.7 = 0.3 / 16.7 = 1.7% off

19.5 - 20 = 0.5 / 19.5 = 2.5% off

23 - 22.3 = 0.7 / 22.3 = 3.1% off

Ok so lowest difference wins with the closest mesh.

So new drive gear it is!


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Old 10-08-2014, 01:46 PM   #19
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http://www.ddperformance.com/content...peedoChart.pdf

Correction you can only go up to 4.30 with the correct speedo gears and a 6 tooth. As you can plainly see there is no Ford gear for higher than 3.27 with the 8 tooth drive gear.
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Old 10-09-2014, 06:32 PM   #20
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Gonna try following this guy's instructions:



Any tips?
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Old 10-12-2014, 08:03 PM   #21
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