Calculating Rear Diff ... check my math!

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7milesout

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Guys,

The last time I went to check the diff size by spinning the driveshaft, it wouldn't cooperate, and I was doing other things and was in a hurry. I didn't have any help either. So I stopped trying and did other things. Then the other day when I was driving it, I confirmed again that right at 60 mph it's turning right at 3,000 rpm.

So, I figured I would do the math to see if I could figure out what diff it has in it. It is a 727 transmission. So the driveshaft is turning 1:1 with the engine (in 3rd). And my tires are 14". The total tire height is really close to 24 inches.

To spare you all the gory details, when I plug in what the speed should be with a 3.23 gear and 24" tires, it comes out to be 66.3 mph. Not close. When I plug in a 3.55 gear I get 60.3 mph. To my surprise, I think I'm running a 3.55 rear gear. The previous owner said he thought it was a 3.23.

My question is, is there anyone running a 3.55 gear with a 727 transmission. If so, are your rpms about the same as mine? What size rear tires?

Thanks.


7milesout
 
I found a tire speed / gear calculator tool online. It seems my math is correct. Would still like to know if others seems to have the same rpm.
 
Guys,

The last time I went to check the diff size by spinning the driveshaft, it wouldn't cooperate, and I was doing other things and was in a hurry. I didn't have any help either. So I stopped trying and did other things. Then the other day when I was driving it, I confirmed again that right at 60 mph it's turning right at 3,000 rpm.

So, I figured I would do the math to see if I could figure out what diff it has in it. It is a 727 transmission. So the driveshaft is turning 1:1 with the engine (in 3rd). And my tires are 14". The total tire height is really close to 24 inches.

To spare you all the gory details, when I plug in what the speed should be with a 3.23 gear and 24" tires, it comes out to be 66.3 mph. Not close. When I plug in a 3.55 gear I get 60.3 mph. To my surprise, I think I'm running a 3.55 rear gear. The previous owner said he thought it was a 3.23.

My question is, is there anyone running a 3.55 gear with a 727 transmission. If so, are your rpms about the same as mine? What size rear tires?

Thanks.


7milesout
3.55 sounds right. I had a car with a BB and a 727 and with 3.23s it ran 3000 rpm @ 70 mph.
 
The torque converter is slipping some, so third gear is not 1:1 to the engine speed. Depending on the load on the torque converter the slip rate will vary. You probably have a 3.23. Assuming an open end diff try putting the car on jack stands, put it in neutral (engine off) and have one wheel held still. Then rotate and count the revolutions of the drive shaft while watching the spinning wheel rotate one revolution. A 3.23 should be 1.615 (just over 1 and half) revolutions of the driveshaft (3.23/2).
 
Unless your converter is really inefficient you have 3.55 gears by my math and that figures about 6% slip.

Also, you have to verify the speedo reads correctly.
 
First thing you must do:

"Also, you have to verify the speedo reads correctly."

Either that, or use GPS/Smartphone to check actual speed @ given RPM, and go from there. I've never seen an old speedometer that was all that accurate.
 
Guys,

The last time I went to check the diff size by spinning the driveshaft, it wouldn't cooperate, and I was doing other things and was in a hurry. I didn't have any help either. So I stopped trying and did other things. Then the other day when I was driving it, I confirmed again that right at 60 mph it's turning right at 3,000 rpm.

So, I figured I would do the math to see if I could figure out what diff it has in it. It is a 727 transmission. So the driveshaft is turning 1:1 with the engine (in 3rd). And my tires are 14". The total tire height is really close to 24 inches.

To spare you all the gory details, when I plug in what the speed should be with a 3.23 gear and 24" tires, it comes out to be 66.3 mph. Not close. When I plug in a 3.55 gear I get 60.3 mph. To my surprise, I think I'm running a 3.55 rear gear. The previous owner said he thought it was a 3.23.

My question is, is there anyone running a 3.55 gear with a 727 transmission. If so, are your rpms about the same as mine? What size rear tires?

Thanks.


7milesout
I've spent the summer experimenting with my rear gear and this is what I documented.

340ci with A727 - BFG 235/60/15, 26" tire:

RPM @ 60 Mph (speed check w/GPS)
  • 3:91 = 3200 RPM
  • 3:55 = 2800 RPM
  • 3:23 = 2500 RPM

AlV
 
Unless your converter is really inefficient you have 3.55 gears by my math and that figures about 6% slip.

Also, you have to verify the speedo reads correctly.

Done that. Both GPS and the "Your Speed Is" signs put up along the sides of the road. When my speed shows ~58.5 mph, I'm actually going 60 mph. And at an INDICATED 58.5 mph is when it is hovering right at 3,000 rpm.

I'm interested in this whole gearing discussion because ... I have an open diff. The open diff will go to the scrap heap ASAP. So, I'm trying to figure out what ratio to go back with. Also in consideration to the ratio, at some point in the future I hope to go to a 4 speed auto. Which one, is undecided, but I think I will try to stick with a mopar trans.
 
Well guys - Regardless of all my calculations, it appears I actually have a 3.23 rear diff. While up under the car working on replacing the fuel sending unit, I spun the rear wheel and counted the driveshaft revolutions. When I spun the LR wheel forward, the RR wheel would not turn. And the driveshaft spun quickly.

When I turned the LR wheel in reverse, the RR wheel would turn the opposite direction and the drive shaft would turn half speed. So, to simplify this for me, I just turned the LR wheel forward.

I started with the driveshaft in a specific position that was easy to remember and easy to know where that part of the driveshaft was at all times. Then I marked the start position on the tire. I spun the LR wheel 1 revolution exactly, and the driveshaft turn just barely less than 3.25 times. I think it would be clearly distinguishable if it turned just over 3.5 times versus just under 3.25 times. So, with that, I have a 3.23 rear end.

Since I'm going to purchase a Detroit / Eaton TrueTrac diff, I'm certainly not going to buy a 3.55, since it turns such high rpm as is.

Comments, questions, suggestions and concerns are welcomed and appreciated. Thanks!


7milesout
 
I use a beat up old GPS for a speedo.
Find and use "such as" the Wallace speed/ gear calculators

Wallace Racing - Automotive Calculators
As others have said, the TC slips

EASY to figure rear gear.......

SAFELY block and jack up the car one rear wheel. In neutral see if it turns. If so, you do NOT have a limited slip/ suregrip rear axle.

Turn the wheel TWO turns while counting the driveshaft revs. That count will give you the rear gear

A slice more than 4 turns of the shaft: 4.10 /4.11
A tiny bit LESS than 4....3.90
A tiny bit more than 3.5 turns.........3.55
and so on..............

If the tire will NOT turn in neutral..........you have some sort of limited slip. Jack up BOTH rear wheels, turn one of them ONE turn while counting the shaft "as above......."
 
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