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I'm looking at purchasing a 2008 CTS RWD. From what I could gather from various forums, it sounds like a limited-slip was only included if you purchased the "summer performance tires" package.

I want to make sure that I get the LSD since I'll be driving it in the winter; is there any way to tell from a VIN number lookup or other method whether the car has one?

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You need to find the RPO (Regular Production Options) list on the Service Parts Identification label. Similar to what is pictured below. It will likely be on the spare tire cover or the glove box, but may also be in the door jam or under some other compartment lid.

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Each on of those three digit number/letters indicates an option the car is equipped with on of them will tell you what kind of rear axle is in the car. You can look them up on a site like www.rpocodes.com

As you can see from the example codes I have listed at the bottom that this vehicle has positive traction and a 3.42 gear ratio. I would start by looking up the codes starting with G, but it could be any of them.

You can also ask your local dealer parts department they should be able to tell you by VIN.

For Example:

1TA : AXLE, FULL FLOATING

G80 : AXLE POSITRACTION, LIMITED SLIP

GU6 : AXLE REAR, 3.42 RATIO

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If you have any doubt at all that the RPO code sticker is accurate (like, maybe someone swapped the rear differential), you can tell an ordinary open diff from an LSD by jacking up the rear end of the car, releasing the parking brake, and turning each rear wheel in turn, leaving the other one free to turn by itself.

With an open differential, the opposite side rear wheel will turn the opposite way (backward vs forward). With limited slip, the opposite side rear wheel will turn the same way (i.e., both wheels turning forward, or both turning in reverse).

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As Larry said, look for the RPO codes. The code you are looking for the Limited Slip option is G80. Look here for a list of RPO codes specific to your CTS.

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