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Reference information - Speedo gears

61K views 56 replies 33 participants last post by  Cougar-67 
#1 ·
This is a bunch of speedo gears I purchased at the Louisville, KY swap meet today and I thought it might be useful for those of you who have speedometer error caused by switching to a different size tire diameter or different rear axle ratio. This is the part needed to fix that.

The two pale green colored gears on the right are for a Four Speed transmission. Notice the teeth slant the opposite direction of the ones on the left which are for automatic transmission such as C4 and C6.

The five gears on the left are:
21 tooth (red),
20 tooth (black)
17 tooth (white)
16 tooth (maroon)

The stock application chart uses tire sizes which are useful as a reference but since they are all bias ply 14 inch tires they are not a whole lot of help unless you have reproduction bias ply tires.

To illustrate what works, I have 3.50 gears and 215/70R-14 tires on my green GTE and the speedo is accurate. It has the black 20 tooth gear installed.

I have 2.75 gears in the blue GTE with 215/70R14 tires in the blue GTE and the speedo is accurate with an 18 tooth gear.
 

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#28 ·
I am putting the speedo system together and I need to figure out the color of gear to run. Here's the problem. I have the reducer, and don't see any refference to adding this into the formula. Stats:
3:91 gears
245-6-15 tires
internal black gear
Speed reducer=
What color gear??
 
#30 ·
Gear

21 tooth.
 
#31 ·
I still buy the gears from Ford for reasonable. There are excellant charts in the chassis assembly manuals on speedo gears
 
#32 ·
Speedo correction how-tos for those who need it (long)...

...I was discussing this on the 429 Ponycar forum and thought it may be of some use here as well. It's long, but worth it if you need it or are just curious.

First off, the way to get the whole system to work right is to make the speed cable rotate 1,000 revolutions for every mile the car is driven. This is done by adjusting the transmission driven to drive gear ratio on the speed cable to match the tire size and rear axle gear ratio of the car.

The first thing to know is how many revolutions the tires make in one mile. To calculate this, measure the diameter of your tires. 26" is about average for F60-15's or 245-60-15's. Multiply this measurement (26") by pi (3.1416). This will give you the circumference of the tire. Next, divide one mile in inches (63360) by the circumference of tire (63360 divided by (26 x pi)=776). This equals the revs per mile of the tire. Now multiply this number by the rear axle ratio to give you the number of drive shaft revs per mile (776 x 3.91= 3033). Next, divide this number by 1000 (3033 divided by 1000 = 3.033). This is the ratio of transmission driven/drive gears on speedometer cable you will need in this case use a driven gear with 18 teeth and a drive gear with 6 teeth (18 divided by 6 = 3.0). This is the best choice which yields a 1.1% high error which means your odometer unit indicates 1.1% greater than actual or 10.11 for an actual 10.0 miles. If you are using the Ford adapter/reducer (reducer was factory equipped on cars with 3.91, 4.11 or 4.30 rear axle ratios), multiply the number by 0.75 (3.033 x 0.75=2.274). This is the driven/drive ratio you need. A 3.75% error (i.e., a 0.375 mile error in 10 miles) is considered acceptable by design.

When picking a driven/drive gear ratio to match the tire and rear axle gear ratio you have a choice of drive gears and driven gears.

The drive gear is inside the transmission and the tail housing must be removed to get at it. You may be able to see what gear your transmission has by looking in the hole where the speedometer cable goes with a light. Manual transmissions have a plastic gear pressed onto the output shaft. The manual trans drive gear is also color coded: pink (6 teeth) or black (7 teeth). Toploaders use the "type 4a" drive gear while automatics use a gear machined to the output shaft with either 8 or 9 teeth. Automatics have a metal gear machined into the output shaft. To change the latter, the whole output shaft must be changed, a big job.

The driven gear clips onto the end of the speed cable. These plastic gears are unique between the manual Toploader transmission and the gears in an automatic transmission. The uniqueness is due to the helix direction of the gear teeth. This is the curvature as seen when looking at the gear teeth. The automatic transmissions are right hand helix (known as type 3). The speedometer cable enters the transmission on the driver's side of the car. The Toploader transmissions with the shift linkage outside of the transmission are left hand helix (known as type 3a). The speedometer cable enters the transmission on the passenger side.

All driven gears are color coded whether they're used with an automatic or manual transmission The automatic and today's manuals are molded in color. The Toploader driven gears are molded in off-white colored plastic and the tips are color coded with paint.

Your choices are (tooth count and color code): 16 (blue for AT, orange for MT), 17 (green AT, purple MT), 18 (grey AT, green MT), 19 (tan AT, pink MT), 20 (orange AT, blue MT), 21 (purple AT, red MT) and 23. The 23 tooth gear is from Saleen and costs $22, but the teeth are thin and the gear will wear out every couple of years. If you cannot make it work with this stuff, you have one more option -- get a reducer box like Ford used with Drag Pak cars. The original is spendy (~$350), but a generic box can be had from a speedo calibration shop for a lot less. I would chose this method over changing the output shaft on an AT unless I had to remove the trans anyway

Here is a handy chart to chose your gear combo after you have done the math shown in the first paragraph. The ranges shown comply with the Ford approved +/- 3.75% allowable error:

Toploader choices:
16/6 = 2.567-2.767
17/6 = 2.728-2.940
18/6 = 2.888-3.113
19/6 = 3.048-3.286
20/6 = 3.209-3.459
21/6 = 3.369-3.631
23*/6 = 3.690-3.978

16/7 = 2.200-2.372
17/7 = 2.338-2.520
18/7 = 2.475-2.667
19/7 = 2.610-2.814
20/7 = 2.750-2.964
21/7 = 2.888-3.113
23*/7 = 3.163-3.409


C4 & C6 choices:
16/7 = 2.200-2.372
17/7 = 2.338-2.520
18/7 = 2.475-2.667
19/7 = 2.610-2.814
20/7 = 2.750-2.964
21/7 = 2.888-3.113
23*/7 = 3.163-3.409

16/8 = 1.925-2.075
17/8 = 2.045-2.205
18/8 = 2.166-2.334
19/8 = 2.286-2.464
20/8 = 2.406-2.594
21/8 = 2.527-2.723
23*/8 = 2.767-2.983

16/9 = 1.711-1.845
17/9 = 1.818-1.960
18/9 = 1.925-2.075
19/9 = 2.032-2.190
20/9 = 2.139-2.305
21/9 = 2.246-2.420
23*/9 = 2.460-2.652
* = Saleen part, not Ford part

Hope this helps!
 
#33 ·
Is it safe to say, that if I had a blue, 16 tooth gear, that it's a D0AZ-17271-A? I pulled this out of a 67 Cougar with no engine or tranny, so I would suspect it was a transplant at one point. Here is a pic


Rick
 
#35 ·
To John Johnson

John,

Your car will probably like the 21 tooth gear. The C4, C6 and FMX all use the same gear.
 
#39 ·
Nope. Still

Art said:
Have these been discontinued by Ford?
(IIRC)
 
#40 ·
Nope. Still available anywhere Ford parts are sold.

Art said:
Have these been discontinued by Ford?
(IIRC)
 
#42 ·
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.
 
#46 ·
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.
 
#47 ·
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.
 
#49 ·
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?

.
 
#50 · (Edited)
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

 
#52 ·
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:
 
#53 ·
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
 
#57 ·
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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