Replacing the driven gear on a 47 car
 

I think I need to replace the speedometer driven gear on my 47 Ford Coupe. Does anyone know if that can be done without dropping the drive shaft. I don’t see a way to get to it.

Re: Replacing the driven gear on a 47 car
 

I assume by "driven gear" you mean the one in the turtle near the front of the torque tube. Even though it is in a blind spot from the underside, the turtle can be easily removed from underneath. First unscrew the knurled nut that holds the speedometer cable to the turtle, pull the cable out of the turtle and move it aside.

With a short 7/16 box end wrench you can feel and unscrew the two short bolts that hold the turtle to the drive shaft. There is a thin gasket so you might need to tap the turtle to get it unstuck from the gasket.


The turtle may have a number cast onto it, probably 18, or 19, depending on what rear end gear ratio the car was delivered with. That number corresponds to the number of teeth on the driven gear in the turtle. I think the 19 tooth goes with 3.78 rear ratio and 18 tooth for cars with 3.54 rear end gears. The turtle housings are ever so slightly different because the number of teeth affects the gear diameter, the 18 tooth being slightly smaller diameter than the 19 tooth. But I am guessing you could use a 18 tooth gear in 19 turtle with no change. To put a 19 tooth in an 18 housing, maybe use two or three gaskets.



How do I know all of this? I switched from 3.78 to 3.25 rear end gears and needed to go to a 17 tooth speedometer gear, as used on some Model A cars. I had a 19 turtle and with the smaller diameter 17 gear, the teeth did not fully engage, causing erratic speedometer. I was able to file down the 19 turtle enough to get the 17 tooth gear to engage.

Re: Replacing the driven gear on a 47 car
 

And this is why you must count the teeth on any turtle you plan to install. Don't assume the number on the top is true ...But, what makes you think your turtle needs replacing? Other than to change ratios, they are relatively trouble free.

Re: Replacing the driven gear on a 47 car
 

Thanks. I will see if I can find and get to the bolts. I’m trying to do this without a lift so it’s awkward to get to.

I think the sleeve where the cable goes into the turtle is rounded out. The speedometer and/or cable were frozen for several years and I think the sleeve got rounded out. The speedometer is fixed now but the cable stops turning shortly after I replace it. It gets slightly rounded so I think it is not seating tightly in the sleeve.

Re: Replacing the driven gear on a 47 car
 

Probably no way to repair the rounded out sleeve. Could buy a turtle from Ebay.

Maybe try this "Farmer Fix" first.
If you can still find a flat wood tooth pick (or make something similar), stick the wide end of it into the sleeve as a shim, break off the excess length, and then force the cable into the sleeve with that shim to tighten the connection of the cable too the sleeve.

Re: Replacing the driven gear on a 47 car
 

Thanks. Good idea.

I see that you are in Williamsburg. I am in Richmond. Do you know anyone who does good body and paint work or interior work in our area? Sorry for adding this to a general message but I am too new to this system to send a PM.

Re: Replacing the driven gear on a 47 car
 

I have never had any body or interior work done here. The body on my '41 is solid, but the paint is "cheapfake". I looked into having it painted. At the price they want now at a classic car shop (> $15K) I would never take the car out of the garage. So, sorry, I have no recommendations. There are others on this board in VA that might.