Technical Torque tube speedo drive questions

I am working on the P-12 build. got the torque tube and drive shaft cut down and am now trying to assemble parts for the speedo drive and I have a few questions.
1. The drive gear on the drive shaft; Were they all the same or are there different tooth counts?
2. The speedo drive housings are stamped with 18, 19, etc. The best that I can tell the 18 goes with the 3.54, the 19 with the 3.73... I have a 4.11 gear set (marked 9-37) should I be looking for a speedo drive marked with a 20? Is there such a thing?
3. The rear tires are 7.50x16... not a factory size, so does this make all the other questions irrelevant?
I spoke with Rusty at Speedometer Service Co. and he thinks he can put together something to correct the speed for my 1000;1 speedometer.
Just figured I would try and get it as close to correct as possible from the start.
thanks for the info.
Chappy

 

The gear on the driveshaft are all the same. I believe the 4.11 is a 21 tooth. I took the time to find a 18 tooth for my last 40 ford, because it had 3.54 gears, but also 7.50 16 tires made it off. So then I bought a GM Stewart Warner speedometer corrector and just guessed on the ratio it was a hassle, and it wasn't correct in the end, but close.

For my current 39 Ford I hit the "EASY" button and called up a guy who advertises in the back of Streetscene. He asks what speed the speedometer says, and then the actual speed. He makes a speedometer corrector to order. The speedo in my 39 is now dead nuts accurate. I think it was $140 but worth it to save all the other messing around.

If Rusty is going to make a corrector for you, don't waste you time messing with it, just slap it together and the corrector will fix it.

 

Yes, the gears on the shaft are all the same. They get the different ratio by bigger/smaller gear in the turtle, so shaft height changes with gear height and you need to change the whole turtle, not the gear only. There is a chart for tire size/rear end ratio that tells what turtle. I believe its at vanpelts

 

Learn something every day.
I replaced just the actual gear inside the turtle to the correct pinion size but couldn’t get it to work.
Now it seems the turtle housing size doesn’t allow it to engage the shaft gear.
Back to the drawing board.