Lazy speedometer

[Ricosan]

OK, let me see if I have this straight. I will need to remove the transmission cover and then mark the gear on the torque tube that is exposed after removing the turtle. I count how many rotations this gear makes for one revolution of my rear tire.
I’ll count the number of teeth in the turtle.
I’m on it! I can’t do it today since I’m taking my wife for a ride this afternoon but Monday looks good.

Richard

[glennpm]

You may not have to take the transmission cover off although it will give you more light. The turtle cutout is on the side of the torque tube so you'll need to figure the best access to mark and count the axle rotations.

[V8COOPMAN]

Originally Posted by Ricosan

OK, let me see if I have this straight. I will need to remove the transmission cover and then mark the gear on the torque tube that is exposed after removing the turtle. I count how many rotations this gear makes for one revolution of my rear tire.
I’ll count the number of teeth in the turtle.
I’m on it! I can’t do it today since I’m taking my wife for a ride this afternoon but Monday looks good.

Richard

Richard.....The pictures below are of a 1935 & a 1940 Ford, but the idea is the SAME. Keep in mind that your '33 FLOOR opening (for access) will probably be somewhat different. These pics might help you pre-plan your mission.

Coop

[glennpm]

I don't think what I posted #40 will work since the end of the snap ring may not be visible after you turn the wheel two times. The other rear wheel must be on the ground.

Pick either the end of the snap ring or other point on the driveshaft gear that you can use as as an indicator. Line it up with the edge of the turtle opening then make a mark on the tire at the bottom perpendicular to the ground.

Rotate the jacked wheel two times. Now rotate the rear wheel slowly until the end of the snap ring or other indicator is even with the edge of the opening. The rear wheel is now rotated some fraction of a full rotation. Divide that total wheel rotation number by 2. The result is then divided into the rotations of the driveshaft giving you the gear ratio.

Example for a 4:11:

- 2 wheel rotations
- snap ring or other indicator is not visible
- rotate wheel again slowly until the driveshaft has rotated 5 times with the indicator even with the opening
- Wheel has been rotated 2.43 turns to accomplish this

Rear end ratio = 5/(2.43/2) = 4.11 (I cheated to make it work out :-)

Hope you get the idea.

[ford38v8]

Driveshaft rotation will match engine rotation when transmission is in direct drive. Position of the fan blade will indicate engine position.

[glennpm]

Sponsored Links (Register now to hide all advertisements)

So with 5 rotations of the driveshaft with snap ring end as indicated in post #44, the tire mark would be in these positions for the three most likely rear-end ratios.

4.11 ---> 2.43 rotations
3.78 ---> 2.65 rotations
3.54 ---> 2.82 rotations



Glenn

[Ricosan]

OK, I think I got it. I removed the transmission cover and the turtle. I had my brother slowly turn the right rear wheel for 2 revolutions and then slowly approached the .43 and it came up on the gear part I had selected to watch. It looks like 411 rear end.
I counted the number of teeth in the turtle - 24.
Glenn, I was unable to download your chart. Will that help determine which turtle tooth count would work with a 411 rear end.

Richard

[glennpm]

Great!
You'll need either a 20 or 21, most likely a 20. The turtle casing you have with a 24 won't work. I believe that for a casing you can only go with a two tooth difference.

Exactly what over or under speed were you at what MPH?

[Ricosan]

When my speedometer registers 35mph my actual speed is 45mph.

I should be looking for 20 or 21. I think I saw one at third gen.

Richard

[glennpm]

Excellent! A 20 tooth should be pretty close.

[Ricosan]

Found a nice looking used 21 tooth turtle in the swap area. The ad was from last year but just thought I would give a holler. He still had it! Hopefully it will come quickly. I’ll post the results.

Richard

[glennpm]

so how have you made out with this Richard?

Glenn

[Ricosan]

Got the 21 tooth turtle installed this morning. Took it out for a test. I am showing a registered speed on my speedometer of 35, actual speed is 41. I came down from 24 teeth to 21 teeth and I got about 4 mph improvement. Maybe a 19 or even an 18 tooth would put me on the money. Thoughts?

Richard

[Ricosan]

I just hit me. My ‘34 had a 411 rear end and 17 X 5.50 tires. Same as my ‘33. I went back through my photos and found a pic of the turtle. It’s a 21 tooth and the speedometer was always spot on.
So now the question is why am I reading 5 or 6 mph off with the same setup? Is it possible that I misread the rear end ratio?
I’m not sure if it’s significant but the needle is not as steady with this new turtle.

Richard

[glennpm]

Boy Richard, I expected that you would be closer to true speed than you are.
It could it be that someone replaced the gear in that "21" you have with a higher count gear? I'd pull the turtle again and count the teeth. I thought a 20 tooth gear would be correct but perhaps a 19? I know that Bratton's sells nylon gears for the turtle.

Glenn

[glennpm]

You may also have the wrong driving gear tooth count, part numbers 17285;
the one that mounts to the driveshaft but changing that would be a big job.

[V8COOPMAN]

Originally Posted by glennpm

You may also have the wrong driving gear tooth count, part numbers 17285;
the one that mounts to the driveshaft but changing that would be a big job.

I DID NOT realize that there were more DRIVE GEARS than one.

Coop

.

[glennpm]

Yes there are two

- The driving gear is the one that is mounted on the driveshaft

- The driven gear is the one in the turtle.

Until I looked into this today, I didn't realize there were several driving gear options.

[Ricosan]

Glenn, it looks like this setup should be working. Being it’s the exact mechanical ratio as my ‘34. - 411/21 teeth/17X5.50. If after tooth count I get 21 teeth, it would leave only the driving gear as suspect.

Richard

[glennpm]

I agree with you.

If it is the same as your 34 and the driving gear is the bad boy, rather than changing it, I would go further down on the driven or turtle gear teeth. The gears only, like from Bratton's for example, are about 15 to 20$ I believe.