Which speedo gear?

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JackFish
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Which speedo gear?

Post #1 by JackFish » Tue Nov 13, 2018 1:06 pm

I had the gears swapped out in an 8.8 for 3.27 ratio.
The Fairmont has a stock 1978 C4.
Which speedometer gear should I be looking for?
1978 Ford Fairmont station wagon
1978 Ford Fairmont station wagon
Yup, I bought another one.
1996 Chevy Caprice 9C1 (3)
1999 Dodge Ram 2500

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JackFish
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Joined: Thu May 26, 2005 7:31 pm
Location: Winnipeg Manitoba Canada

Re: Which speedo gear?

Post #2 by JackFish » Fri Nov 16, 2018 5:11 pm

I need to know the drive gear that would be in this transmission.
Anyone have any idea?
1978 Ford Fairmont station wagon
1978 Ford Fairmont station wagon
Yup, I bought another one.
1996 Chevy Caprice 9C1 (3)
1999 Dodge Ram 2500

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xctasy
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Re: Which speedo gear?

Post #3 by xctasy » Mon Dec 10, 2018 3:04 am

JackFish wrote:I need to know the drive gear that would be in this transmission.
Anyone have any idea?


Driven gears need to be 16, 19 or 21 depending on if you have 6, 7 or 8 teeth drive wheel.

But 19 oughta do it, because your've probably got a 7 teeth drive wheel. :mrgreen:

A yellow 7-tooth drive gear is correct for a T5 or 3-speed, but a 7 tooth drive gear for a Toploader
4-speed, or an SROD is cut wrong.


If its C4 or C3, its got the short nose drive

here's a pic of an AOD gear beside some T-5 gears. Note the longer snout on the AOD gear. T5 and C4/C3's interchange.

Image


The stock setup was 2.73 with the optional 3.08 and most were the 3.08 gears.

Ford used a seven ( 7) tooth drive gear in the 1983 to 1989 T-5's,
then switched to an eight ( 8 ) tooth drive in 1990 and used through 1995.

3.3 for 1978 with C4 was always either a 6.75 or 7.5 in 2.73 or maybee the 3.08.

Speedo drive gear with 185 78 14'S would be right hand C4. The speedo gears are 6, 7 or 8,

6 E3ZZ-17285-B Black
7 E3ZZ-17285-A Yellow
8 F0ZZ-17285-A Green


the driven gears are from 16, 17, 18, 19, 20, 21. But they go up to 22, 23 or more.

16 C0DZ-17271-A Wine 16 D0AZ-17271-A Blue
17 C3DZ-17271-C White 17 C7SZ-17271-A Green
18 C0DD-17271-B Yellow 18 C7SZ-17271-B Gray
19 C0DZ-17271-B Pink 19 C7VY-17271-A Tan
20 C1DZ-17271-A Black 20 C8SZ-17271-B Orange
21 C4OZ-17271-A Red 21 D0OZ-17271-B Purple
23* DFM-17271-A White

Image

http://www.v8guy.com//interestingstuff/ ... Chart3.jpg


Toploader and SROD gears have a left-hand slant (\\\\)
The C4, AOD, and T5 style gear has a right-hand slant (////).

Calcualtions:

See http://vb.foureyedpride.com/showthread. ... gear-drive
http://vb.foureyedpride.com/showthread. ... edo%20gear

Tire Rev's Per Mile are based on the static tire height less a 2 to 3% loaded wheel reduction. 800 to 815 is common for most Foxes.

Tire Rev's Per Mile calc:-https://www.miata.net/garage/tirecalc.html

Section Width / 25.4 x (Aspect Ratio / 100) x 2 + Loaded Wheel Diameter = Tire Diameter

Tire Diameter x Pi = Tire Circumference

Feet per Mile x 12 = Inches per Mile

Inches per Mile / Tire Circumference = Tire RPM

http://vb.foureyedpride.com/showthread. ... peedo-Gear
Example with 827 revs per mile tire

Drive Gear Teeth X Axle Ratio X Tire Rev's Per Mile / 1000
6,7,or 8 X 3.27 X 827 /1000 = these numbers below

6T= 16.22 .22 over original Number of 16
7T= 18.93 .07 under original Number of 19
8T= 21.63 .63 over original Number of 21
Image
XEC Ltd ICBE's Inter Continental Ballistic Engines-
FAZER 6Bi (M112 & EEC5) or FAZER 6Ti (GT3582 & EEC5) 425 HP 4.1L/250 I-6
FAZER 6V0 3x2-BBL Holley 188 HP 3.3L/200 I-6 or 235 HP 4.1L/250 I-6
X-Flow Engine Components Ltd http://www.xecltd.info/?rd=10

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JackFish
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Re: Which speedo gear?

Post #4 by JackFish » Mon Dec 10, 2018 1:06 pm

Well holy moly information overload X, as usual. :lol:
Thanks very much for the input.
I did find a solution but didn't post.
I determined I have a red gear in there now so it's 21 teeth.
The speedo was out about 25% as in it reads 125kmh when traveling @ 100kmh gps.

I found a formula that verified the correct math and the closest gear would be the 16 tooth/purple.

I just happen to have a purple one somewhere in my stash but have to dig it out.
So I'll give that a go and verify.
But then it's freakin' cold out there right now and I also need to get a round tuit. :rolflmao:
1978 Ford Fairmont station wagon
1978 Ford Fairmont station wagon
Yup, I bought another one.
1996 Chevy Caprice 9C1 (3)
1999 Dodge Ram 2500

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