Turtles!

Bassman/NZ

Bassman/NZ

Well-known member
Just got my car on the road and the speedo is reading about 8 mph slow at 30mph, probably due to the 7.50x16 tires on the back. Gearing is 3.78, with the tires it's probably up to around 3.54 at a guess. I believe my turtle is 19 tooth, but I cant get my head around whether I need to go to a 21 tooth or 18 tooth turtle. Help! :confused:
Also, does the gear on the driveshaft need to be changed as well?
 
I believe you'll need a 18 tooth. Chart doesn't have 7.50x16"s on it. But 7.00x16 with a 3:78 it recommends to use a 18. The 18 will mesh but you will need the bigger 18 turtle shell. At least that is my understanding.

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The drive gear remains the same. Turtles are specific to the driven gear, and show the tooth count in the casting on top. However, I've been told that they can and are modified to accept different gears, which are easier to get hold of than are complete turtles.
 
A machine shop just down the road a piece from me makes the reproduction driven gears in all numericals. I do not know who he contracts with to make them for. They are made from high impact plastic and will outlast one of the original gears by a long shot.
 
The FordBarn is a wonderful place. One of our esteemed members has contacted me privately and offered to send me a complete 18 tooth turtle. Life is good!
 
Bassman I thought I could help you with the 18 turtle but can't. I suggest you take yours off and see what is on there. Also double check ratio of the rear then we can try to figure out the turtle you need. Shipping is going to cost about $25.00 so it would be good to get the right one the first time. Does anyone else reading this post have one to help him out?? Look under the work bench there has to be a lot of these laying around. G.M.
 
Thanks GM. On reflection, I'm not sure I'm barking up the right tree. My speedo needs to spin faster to get the reading in the ballpark. Therefore would not the higher numbered turtle do the job? I'm confused with this, and need someone whose had this experience to chime in. I currently have a 19 turtle, so do I need an 18 or a 21, or even a 23?
In essence, does the higher number mean the cable spins faster?
My head hurts..... :(
 
Think of it this way... if you got mad at your wife and parked the car to walk around the block, you'd get back quicker if the block was smaller! (Ask me how humongous that durn block was!)

The teeth are the same size, just more of them on the bigger gear, so you want a smaller gear to run up the RPMs on the cable.
 
I have only seen 3 different driven gears, there may be more. 18 tooth for the 3:54...19 for the 3:78...21 for the 4:11 tire size comes into play as well... some times you have to experiment
 
32 Phil,I have had recently a NOS 17 tooth& turtle.The 17 was stamped in ,not embossed.Its for 3.25 gears I believe.
 
I have seen charts published on the speedo drive gear, rear ratio and tire size. I believe it's towards the back of the green chassis parts book. I have a bucket full of brand new 23 turtles. G.M.
 
A friend put 3.54 ,s to replace the 3.78,s & installed an 18 tooth gear.The speedo wouldnt work & we discoveredthe new gear was smaller diameter than the 19 tooth gear.Iread somewhere on the ,net that the 18 tooth gear takes a turtle w/a slightly different mounting radius to move gear closer to DS gear It would be something to check out.If need be, you could grind a bit off the hsg to make it fit.Make sure you have a slight bit of back lash in the gear when its snugged up.
 
Discovered Brattons are selling 18 and 17 teeth gears. This might be my path if I cant find a complete turtle, bearing in mind ColumbiA 's post above.
 
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