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Royce, you have been a cornacopia of knowledge, to me. But cougars unlimited sell the same gears, no hassel. I currently run a red 21 teeth. 16 in. rim, low profile tire ,3:89 gear in the back of a stock four speed, about 5mph off at 65mph
 

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a crapshoot, i installed a 20 tooth gear into my car after installing 3.5 gears in the rear. with the Nitto tires being just under 26 inches, its still a little off. after reading up and actually doing the calculations, the 21 tooth looks like the best bet. i guess ill call the Ford dealer tomorrow.

anyone know what the tranny most likely would have come with as the drive gear in a typical 69 Cougar FMX with a 3.0 gear? i cant imagine it varied THAT much. my exhaust is in the way of being able to look into the tranny to check it out myself. the car had a tan 19 tooth gear stock though.
 

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This is great info!

Two questions:
1) How can I tell which drive gear I have in my '68 C4? (Color coded? Are they all the same? If so, which is it?)
2) Royce mentioned that the speedo itself can become less accurate though the odometer will be dead-on (if the gearing is set properly) - is there any way to repair the speedo inaccuracy?


My speedo is goofy... reading 80 mph at 60, but 45 at 40 - i.e. non-linear relationship - I'm sure the speedo gear is wrong, but that should be a fixed ratio error. If 45 at 40, then 90 at 80 -- not 90 at 65.
 

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This is great info!

Two questions:
1) How can I tell which drive gear I have in my '68 C4? (Color coded? Are they all the same? If so, which is it?)
2) Royce mentioned that the speedo itself can become less accurate though the odometer will be dead-on (if the gearing is set properly) - is there any way to repair the speedo inaccuracy?


My speedo is goofy... reading 80 mph at 60, but 45 at 40 - i.e. non-linear relationship - I'm sure the speedo gear is wrong, but that should be a fixed ratio error. If 45 at 40, then 90 at 80 -- not 90 at 65.
My speedo reads 2-4 mph slow depending on the speed. However, the odometer loses only about .10 of a mile, every 5 miles. I'm using a 19 tooth speedo gear (AOD trans), 3.25 gears, and 225/55 16 tires in the rear. The gear for this combination actually doesn't match what I'd originally calculated, about 21 teeth. After the Tachman fixed my speedo, the speed indicator was way off, and I had to steep down.
 

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Is my math correct?

Am I doing this calculation correctly?

Numbers I used; (I referenced this formula from Jegs website)
C4 Transmission Drive Gear = 6 (is that right?)
Tire = 235/60R/14 = Diameter of approx. 25.10"
Rear = 3.55
Tire Revolutions/Mile = 20168/25.10 = 803.5
No. of Driven Teeth = (6 X 3.55 X 803.5)/1001 =
17.09

So if I did this calculation right then I would need to purchase a 17 tooth drive gear.

Do you see any errors here in my cyphering or guzzintus or the formulas I used?

.
 

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WCCC a resource site?

Joshua (our full time tech guy) has been working diligently for some time on this project. He gleaned all he could from this thread, the MPC and other resources on the net. To our knowledge there is no equal on the web. Try it out, let us know if you find any bugs or if you have an idea on how to improve it. This is the first of many such projects we would like to have on our site in time.



Online Store :: Resources :: Calculators :: Speedometer Gear Calculator

Speedometer Gear Calculator

 

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Adding to the knowledge-base, here is a picture showing a long snout (green) and short snout (white) gear. The white one is a Scott Drake repro, 20 tooth. The Green one has 17 teeth and was in the transmission when I got the car (car had non-stock wheels and rear gear, which I am returning to stock configuration).

So... these aren't interchangeable, right?





Royce,

One thing you failed to mention is the fact that there are "short" and "long" gear snouts. Yours appear to be short. The long are needed for FMX,C6,AOD on a factory automatic car.

A factory 3 speed car uses the same rotation gear but a different speedo cable and different length snout gear.

A factory 4 speed car uses the reverse rotation (shorter?) gear.

The gear sections are identical in length -- the snouts change.

Been down this road already!
Bill,

In 1967 - 68 there are no Long Snout gears.

The ones shown are correct for C4 / C6 or (opposite rotation) 3 and 4 speed manual trans for 1967 - 68.

Looking in the Lincoln Mercury Master Parts Catalog, Page 2 of section 172 we see that all the gears I have can be identified as either Type 3A or Type 3C. These are short snout. Further investigation of a specific application shows:

Page 2, Section 172.66
Application = 1968 390 / C6Auto
Tire Size = 8.45X15
Gear Ratio = 3.25
Driven gear P/N = C0DD 17271-C
Back to the identification tables, this gear type is 3A, 19 tooth short shaft. A quick look at the application page reveals the same gear is usedwith a C4 application. Notice the tall tire size, the chart is poor when it comes to selecting a gear to use with Cougar sized 14 inch tires.

I had never seen a long snout gear in person, only the illustration in the MPC. I reverse cross checked one of the part numbers and found it is used in later Cougars with automatic transmissions. This would necessitate internal changes in the transmission so a later transmission in a 67 - 68 would need this also.

The long snout version (Type 3) is used beginning in the 1969 model year in Cougars and other Ford / Mercury Products with automatic transmission. It appears that the introduction of the FMX transmission might be the reason, I don't know for sure. Three and Four Speed manual transmissions continue to use the short nose gear even in later years according to the Master Parts Catalog.

So with regard to 69-70 model year Bill you are correct, the long snout gears are for automatic applications.

I learn something new every day!
 

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which one

I have a 67 cougar 289 engine I swaped from a c4 to a 4 speed 68 topload I have the stock rearend 8" which installed a posi I"m going to use 15" wheels rather than the 14" There is no tag on the rearend so I"m not sure what the differential gear ratio is I need to find out so we can find out the speedogear size {how many teeth} anyone got any ideas would like to hear from you THANKS:grin34:
 

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I have a 67 cougar 289 engine I swapped from a c4 to a 4 speed 68 toploader I have the stock rearend 8" which installed a posi I"m going to use 15" wheels rather than the 14" There is no tag on the rear end so I"m not sure what the differential gear ratio is I need to find out so we can find out the speedo gear size {how many teeth} anyone got any ideas would like to hear from you THANKS:grin34:
Just click onto this link!

http://www.cougarpartscatalog.com/speedocalc.html
 

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NICE!!!

Who wants to machine a 23 tooth gear for me now? lol
Really, I'm running a 21 tooth gear and still show too high of a mph. Think I'll just live with it... 3.55 rear end gears. Sheesh...
 

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I do wear a speedo when swimming, but never noticed any gears, what gives? :buck:
 

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Here's a straightforward site I just used to calculate tire height/diameter - https://tiresize.com/height-calculator/

I love my Firestone Firehawk SS's from the early 90's. Had 205/70/14's all the way around, but wanted a meaner looking rear tire, so I went with 235/60/14's. Calculator said my height is 25.1, so plugging that into Don's calculator, along with 3.4 posi from Mustang II and 9-toothed C4, I come up with... 25 tooth gear? BTW, that calculator shows my 205/70/14 as having a 25.3" height. I see a 23-tooth for sale, part #14350. Will that work, kinda sorta?

Originally was a 2.75(?), then I went the idiot route with a 4.11, then after I started tearing everything apart I went with a more manageable 3.4 posi.
 
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