Differential Gearing

MJFUR

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My new to me 69 Chrysler 300 has what looks like the original rear axle, 8 3/4 -489. It's non-suregrip and what I presumed was the standard 2.76.

First pic - chalk mark at 9:00 on tire with a corresponding chalk mark on the bottom of the drive shaft.
As I rotate the tire and watch the drive shaft (both wheels in the air, car in neutral), when the drive shaft makes one complete revolution the chalk mark on the tire is at 6:30. (Second pic)
One complete revolution of the tire rotates the driveshaft approx 1.5 times.

What am I missing here? Doh!

Thx

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You need to make sure only one wheel is turning. You’re wasting some motion through the spider gears driving the other wheel in the opposite direction. That’s my guess!
 
Think for sure grip both wheels need to spin and for open diff only one needs to spin
 
On an open differential you can have both wheels off the ground, transmission in park and spin the wheels by hand. The driveshaft will not move. If you look at the other wheel it will be rotating in the opposite direction. If the transmission is out of gear, some rotation will randomly go to the other wheel and some to the driveshaft.
 
Look at the mounting bolts for the 3rd member, there will usually be a stamped metal tag with the gear ratio attached to one of them. Failing that, the build sheet is coded for the gear ratio.

Dave
 
With a non-limited-slip rear axle, it IS possible to get both rear wheels turning in the same direction, BUT it is tricky from my experiences. Doing a Sure-Grip type is much easier, especially if the differential is a "locked until it unlocks" model. Best to look for that metal tag Davea Lux mentioned.

Reading the FSM as to WOT and min-throttle transmission upshift points, you can also tell which axle ratio is in it. Possibly the better choice would be the 2-3 WOT upshift speed? Then, consider that a 3.23 will be approx 25mph/1000rpm in 3rd, whereas the 2.76 will be approx 28mph/1000rpm in 3rd. Seems like the axle ratio also has a space on the underhood Data Plate?

The good news is that if it's not a 2.76, the only other possibility was 3.23. I can live with a bit less tire smoke to get a car that easily cruises at 85mph all day long with a 2.76, personally.

Just some thoughts,
CBODY67
 
Open diff: I agree with having the rear in the air, both wheels off the ground. Trans in Neutral. Hold one wheel steady with a hydraulic jack or an assistant's help. Turn the driveshaft and see how much the free wheel turns.
 
on a non posi you need to go two full turns of the tire or times your reading of 1.5 x 2 =2.93. you get a better reading going around twice. yes hold one wheel while spinning other.
 
i mark one of the sides of the u joint, use the valve stem at the bottom near the floor as your starting mark. make two turns of the tire and end where you started on the bottom. while turning the tire count the u joint and watch how many revolutions of the u joint.
 
As stated above + the obvious, it can be hard to tell 2.73 from 2.94 with these methods
 
As stated above + the obvious, it can be hard to tell 2.73 from 2.94 with these methods
It’s not hard to tell at all, it’s all in the gears, there are no variables in the gear teeth. The only variable is human. So have the u-joint exactly flat on bottom first, mark the tire. Now start spinning and counting.

Just like testing in science lab, control the variables on your end, because the gears are what they are.
 
Guess I'm the dumb one then advising that it can be difficult to accurately measure especially given the slop in some of these tired out drivetrains.
 
Guess I'm the dumb one then advising that it can be difficult to accurately measure especially given the slop in some of these tired out drivetrains.
not about being dumb or smart, more about doing it a few times and getting experience.
 
Guess I'm the dumb one then advising that it can be difficult to accurately measure especially given the slop in some of these tired out drivetrains.
Those are your words. My post wasn’t written for anyone to read anything into it.

There won’t be .2 worth of slop in the ring and pinion. Maybe if the spider gear pin has egged out the carrier, but that’s pretty rare.
 
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