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Determining if differential is LSD or not.. and gear ratio

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I am trying to determine if the diff in a car is LSD. Jacked the rear wheels off the ground and with car in neutral, turned the wheel on one side and the other wheel did not move. Put the car in 1st gear and turned the wheel on one side and the other wheel turned in the opposite direction. Final crude test was got in the car , did a burnout and the car left two equal black tire marks.

The diff does have a finned cover on it. Also is there any easy way to confirm gear ratio other than pulling cover and counting gear teeth? Car is a square light model. Was wondering if the diff had something cool like a 4.11 ratio installed.


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There should be some stampings on the driver side of the case. You might have to get under the car with a good light and scrape off some crud. But, there will be some numbers and maybe a letter.

For example, if there is "11 S 43" that is a 3.91 LSD. Divide the larger number by the smaller to get the gear ratio. If there is no S, I'm betting it's not an LSD. Also, the LSD would have a white S painted on the passenger side of the case.

The finned differential cover is not necessarily an indicator of an LSD. It's just a late (possibly '76 only if memory serves) standard 02 diff cover. (Of course, it could be an E21 diff cover, but without pictures, I wouldn't know that.)


1972 2002 - 2577652 Follow the fun

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You've described an open differential, sorry.

A '76 should have a 3.90:1, others would have a 3.64:1 (I think). There will be numbers stamped on the left side of the housing, 39-10 for a 3.90:1, 40-11 for a 3.64:1. If the internals have been changed then all bets are off.

The numbers can be hard to read, I used a pencil and paper to make a rubbing of mine.

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Thanks guys. Car has a lot of grunt off the line and is a late model square light example, so very likely a 3.91

I guess from Muscle car days with posi-traction, I thought the two black marks indicated LSD, "posi-traction", "sure-grip" type diff.

Maybe the dual tire marks are a function of the independent rear suspension.

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....and when you remove the diff cover - you will read the ratio


no mystery - no counting needed

and you will see either a open cluster of gears (open diff)

or you'll see a enclosed 'can' looking center section which

contains the clutch pack of the LS .

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

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Guest Anonymous
The best way to tell is to jack up the rear end. put the car in gear. Grab ahold of one of the rear tires and turn it. If the opposite wheel turns:

same direction - LSD

opposite direction - standard

actually this is not quite true

my dif is definitely lsd and the tires turn in opposite directions

also not all difs have stampings

the only way to know for sure is to take the cover off and have a look

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