Questions about the "spin test" to determine differential gear ratio.

[Bart K]

I tried doing the spin test because I wanted to determine the rear end gear ratio of my 68 Super Bee. What I believe I determined is that I have an open diff, but I was not able to determine the ratio because with both tires in the air and the auto trans in "N" per instructions, I could spin the tire several times before the driveshaft would move much at all. Nothing like the expected 2, 3, or 4 driveshaft rotations per one tire rotation. Am I doing something wrong in the test, or is something wrong with the car? The car runs and drives fine, but it will even not spin one tire off the line. Is it possible to have some slip in the diff? I am attaching one pic that shows a number on the 3rd member and when I look it up, Google says this is an 8 3/4 489 case. I am also attaching a pic of the build sheet showing numbers relating to pinion. Any help would be much appreciated!

 

[R413]

3.23 originally. A 1968 superbee should have a 742 housing so this has likely been changed out.

Why is the big block here at the leaf spring and axle connection. Photo one Should not be there. Stock set up shown in photo 2

Why can't it spin a tire? it should smolder the tires easily from a standing start.

The yoke should have a dust shield on it which is missing. It is still shiny and not rusted yet. 3rd photo

 

[Bart K]

3.23 originally. A 1968 superbee should have a 742 housing so this has likely been changed out.

Do you know if the Bee should have had Sure Grip?

 

[Bird 426]

You can do it the other way. Turn your drive shaft and count the wheel revolution. Almost surprised that from the factory that it wouldn't have a sure grip. Looks like your leaf springs are sagging, not sure what that thick piece is between the the u bolt and spring. Someone has had that all outa there at sometime. Maybe it's time for some updating and pull the third member out and see what's actually in there. The broadcast sheet only means what it came from the factory with.

 

[R413]

Sure grip is optional on a super bee with 8 3/4 axle.

 

[Bart K]

You can do it the other way. Turn your drive shaft and count the wheel revolution. Almost surprised that from the factory that it wouldn't have a sure grip. Looks like your leaf springs are sagging, not sure what that thick piece is between the the u bolt and spring. Someone has had that all outa there at sometime. Maybe it's time for some updating and pull the third member out and see what's actually in there. The broadcast sheet only means what it came from the factory with.

You can do it the other way. Turn your drive shaft and count the wheel revolution. Almost surprised that from the factory that it wouldn't have a sure grip. Looks like your leaf springs are sagging, not sure what that thick piece is between the the u bolt and spring. Someone has had that all outa there at sometime. Maybe it's time for some updating and pull the third member out and see what's actually in there. The broadcast sheet only means what it came from the factory with.

I will try it the other way. I have only had the car since December 2025 and have done brakes and had the trans rebuilt (it had to be pulled to fix a bad shaft seal leak) and a few other smaller things. As for the block under the springs, that is how it was when I bought it. I am thinking to pull the third member as you mentioned, I think something is up with it. Since I am not independently wealthy, I have to space projects out a bit. It is not like I go around smoking tires all the time, but I would think with an open diff, this car should be able to spin one tire anyway, but it cannot. Like I said, it runs and drives really nicely, and the kickdown works nicely, so something is up off the line.

 

[Bart K]

Sure grip is optional on a super bee with 8 3/4 axle.

Thanks for the info! As far as I can tell, the car is pretty original, so I do not want to pollute it too much!

 

[khryslerkid]

Open rear...

 

[496 polara]

Is the speedo correct? Do you have an accurate tachometer? If so, what rpm is it turning at 60 mph?

If that rear end has been swapped you could have a 2.76 peg leg in there....or anything else.

 

[496 polara]

Thanks for the info! As far as I can tell, the car is pretty original, so I do not want to pollute it too much!

Original is fine but original or not a sure grip only enhances your driving experience.

 

[33 IMP]

KK's tutorial will answer your questions.
You likely moved the tire some revolutions before the drives haft moved, cause the other tire was turning too, in the opposite directions.
(Sure grip will have both turning the same direction.)
Put one tire on the ground, turn the other two times (because of the 2 to 1 spider gear/side gear ratio) and count the turn of your drives haft.
I have seen tutorials that recommend ten turns of the tire to be more accurate... but I don't think that is necessary.

 

[33 IMP]

Can you (or, have you) done a brake stand? It should boil the right rear, especially if you have a two series ratio.

If it's got a 2.76 or something, spinning tires should be hard. Still, I'd look into the engine timing. Initial timing, total, and advance rate. Might need a timing tape on the balancer.

 

[Bart K]

Is the speedo correct? Do you have an accurate tachometer? If so, what rpm is it turning at 60 mph?

If that rear end has been swapped you could have a 2.76 peg leg in there....or anything else.

Hell, I cannot be sure that anything is correct! I can tell you that the car shows to be turning 3000 rpm at 80 mph.

 

[Bart K]

3.23 originally. A 1968 superbee should have a 742 housing so this has likely been changed out.

Why is the big block here at the leaf spring and axle connection. Photo one Should not be there. Stock set up shown in photo 2

Why can't it spin a tire? it should smolder the tires easily from a standing start.

The yoke should have a dust shield on it which is missing. It is still shiny and not rusted yet. 3rd photo

View attachment 2044295

View attachment 2044414

View attachment 2044415

Ok, I got back out to the garage and did the spin test with one tire on the ground and I get 3 plus a little bit driveshaft rotations with 2 full tire spins, so I see that as probably a 3:23. The block looking thing you highlighted in the pic is just rubber. It is thick on the sides, but thinner under the actual leaf spring.

 

[Bart K]

Original is fine but original or not a sure grip only enhances your driving experience.

Agreed. Since the Bee could have the option of Sure Grip, I will plan on going down that road at some point (when money is saved up).

 

[Bart K]

Can you (or, have you) done a brake stand? It should boil the right rear, especially if you have a two series ratio.

If it's got a 2.76 or something, spinning tires should be hard. Still, I'd look into the engine timing. Initial timing, total, and advance rate. Might need a timing tape on the balancer.

I have tried it (brake stand) and it did not work even though I have verified a 3:23 diff ratio. Maybe I am doing it wrong? Maybe it is due to tire size??? I am running BF Goodrich Radial T/A P275/60/R15. It would probably smoke an OE sized tire.

 

[496 polara]

Have you checked/set the timing ? Any 383 with 3.23 gears should bake one tire easily.

 

[Bart K]

Have you checked/set the timing ? Any 383 with 3.23 gears should bake one tire easily.

Going from memory, at one point the timing was 38 degrees advanced and was changed to something like 18 degrees advanced. Either way, could not spin one tire. Something is not right.

 

[496 polara]

Time to check all the basics including maybe a compression test as well.

 

[Bart K]

Time to check all the basics including maybe a compression test as well.

Agreed! It is a great car and drives well, just nothing off the line.