17 tooth speedometer gear in a 1940

Dan Hay

Dan Hay

Junior Member
I bought a 17 tooth model A gear from Brattons. Car is running, but it looks like the mesh is not correct with my 40 worm gear on the driveshaft. It spins intermittently like its skipping teeth.

Do I need to machine down the housing for a better mesh? Or is an 18 tooth as low as I can go with the stock 40 driveshaft gear?

I have 3.54 gears and 750 16 tires. I bought the 17 tooth thinking that would compensate for the bigger tires.

Thanks in advance
 
I don't know what to tell you about your '40 but I have a 17 gear in a '36 (don't know the gear vendor source) and have no problems.

Tom
 
The housing for the gears vary. You may be able to use a 18 gear in a 19 or 21 housing but not the 17. Also check the Chassis parts books for the proper drive shaft gear to use on the 17. Not long ago Southside obsolete had the entire 17 gear and housing, not sure if he has any left.
 
It sounds like you may have that 17 tooth gear in the wrong turtle housing. What number(s) are cast in on the outside of the turtle housing you are using?
Those nylon gears sold by Brattons are advertised for model A thru '48 Ford and should be direct replacements for the original Ford aluminum alloy gears.

Edit: Also, I just recently installed 3.54 gears in my 35 fordor sedan. I did have an original #17 turtle housing with the original 17 tooth aluminum alloy gear meshing with a '35-'36 inner worm gear on the driveshaft. The worm gear I have should be the same as the one in your '40. This gear set and the 17 tooth turtle gear gives me an accurate mph reading on my original speedometer.
 
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It's in a 19 housing. I have an 18 housing around here somewhere. I didn't realize the housings varied as well
 
So I guess a 17 housing is like hens teeth? Should I round up a more common 18 housing and gear and be done with it?
 
Dan Hay said:
So I guess a 17 housing is like hens teeth? Should I round up a more common 18 housing and gear and be done with it?
Click to expand...

If you have a #18 housing, your 17 tooth gear from Brattons should fit that housing and work well when mounted on torque tube, assuming you have the correct internal gear on the driveshaft. The #17 turtle housings are difficult to find.

If necessary, and I don't believe it will be, you could drum sand the radius area on the turtle housing ever so slightly to give that 17 tooth gear slightly more mesh with the internal worm gear.

On the next '35 rear I build with 3.54 gears, I will have to use a 17 tooth gear in a #18 turtle housing because that's all I have right now.
 
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image.jpg

This is a screen cap of a speedo chart I found elsewhere here. The left column is the speedo gear for a 3.54 ratio rear. The right column is tire size.

I'm running a 750 16, but the closest thing on the chart is a 700 16, which recommends a 20 tooth gear. I was surprised by this because I assumed I would need a smaller gear for the bigger tires.
 
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