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Old 07-12-2020, 10:37 PM   #1
fordblitz
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Default Speedometer gears- 1940's 4 speed truck transmissions

Hello,

While working out my speedometer cable for my 1943 Ford Canada CMP truck I was looking at speedometer gears and found that the 4 x 4 transfer case speedo driven gear is unique to these vehicles and not a standard Ford item.

Just for curiosity I looked at what the standard Ford car and truck gears were and found a list of what was used in the Forties for various tyre and diff combos.

For 4 speed truck transmissions:
An example is 19 tooth driven can be used with 6 or 8 tooth drive gears.
Also 20 tooth driven gears can also be used with 6 or 8 tooth drive gears.

The question I am asking is how can different tooth number speedo driven gears be used on the same tooth size drive gear? The geometry of it doesn't seem possible. I know later GM cars used an eccentric driven gear housing to be able to rotate it to get proper engagement with the drive gear using different size gears. Did Ford do the same?

Found the link on here Ford Barn:
http://i1059.photobucket.com/albums/...rGearChart.jpg

Attached is a commercial 20 tooth gear from a 4 x 2 Ford army truck, and the 15 tooth transfer case speedo gear.

Will be interested to find how it is done as I don't have any commercial 4 speed truck transmissions to view. Thanks.

Cheers
Attached Images
File Type: jpg 01Y-17271-C 20 tooth.jpg (51.0 KB, 1 views)
File Type: jpg CMP 4x4 transfer case end.jpg (76.4 KB, 1 views)

Last edited by fordblitz; 07-12-2020 at 11:36 PM.
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Old 07-13-2020, 12:04 AM   #2
cas3
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Default Re: Speedometer gears- 1940's 4 speed truck transmissions

does it use the standard ford "turtle" that the gear and shaft you show fit in? if so, they change the height of the shaft to use a different size driven gear. if you carefully measured, you would see that a 19, 20, 21, etc. have the shaft location in a different spot, and the gear is bigger/smaller to make it work. you can not switch gears, you need the correct turtle to make it all work
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Old 07-13-2020, 01:02 AM   #3
fordblitz
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Default Re: Speedometer gears- 1940's 4 speed truck transmissions

Hi,

Don't think a "Turtle" is used on the 4 speeds.

Here is a typical mount as used on the 4 x 2 Ford CMP Trucks. Looks the same as the commercial truck rear mount.
No proviso to move the driven gear by changing anything that I can see.

Perhaps they had different rear mounts for different speedo gears? Would not be a really hard changeout.

Just noticed the "2" on the casting. Perhaps some significance?

Cheers,
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File Type: jpg IMG_0225.jpg (81.8 KB, 16 views)
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Old 07-13-2020, 01:15 AM   #4
Tinker
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Default Re: Speedometer gears- 1940's 4 speed truck transmissions

Early fords the turtle was on the torque tube. Interesting stuff added prior.
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File Type: jpg SPEEDO CHART.jpg (188.1 KB, 10 views)
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Old 07-13-2020, 01:41 AM   #5
fordblitz
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Default Re: Speedometer gears- 1940's 4 speed truck transmissions

OK, I understand the different turtles, and the torque tube models, but it still doesn't explain how the 1942 to 1947 4 speed truck gear chart speedometer gears combinations worked with a fixed speedometer gear entry point on the wartime rear transmission mount.

This transmission has a yoke and the U-joint at the rear like any modern transmission.
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Old 07-13-2020, 05:21 AM   #6
flatheadmurre
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Default Re: Speedometer gears- 1940's 4 speed truck transmissions

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To a limit you can use the same gear against different ratios gears...difference is how deep they engage.

So you will be moving the contact surface further in or out on the drive gear.

Any "modern" gearbox use the same technology so not uncommon.
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Old 07-13-2020, 05:43 PM   #7
fordblitz
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Default Re: Speedometer gears- 1940's 4 speed truck transmissions

Quote:
Originally Posted by flatheadmurre View Post
To a limit you can use the same gear against different ratios gears...difference is how deep they engage.

So you will be moving the contact surface further in or out on the drive gear.

Any "modern" gearbox use the same technology so not uncommon.
Thanks,

So I am only guessing from this that Ford designed the 4 speed transmission mount so the driven gear was at the maximum distance required by the various combinations of drive and driven gears for the speedometer and just allowed less than full engagement of the gears wherever else it was required. Doesn't seem like good engineering practice, but I guess whatever gets the job done is OK. Easier than keeping a number of different type of transmission mounts on hand despite the fact they did that with the "turtles". Might be a good exercise to sit down with the various gear tooth combinations and work out the centre to centre distances of the gears to see how much variance there is.
From an engineering point I prefer the GM adjustable eccentric mount for the driven gear.
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