Is "posi-traction" just another name for "limited slip differential"?

Dear Car Talk

Dear Car Talk | Jan 01, 1993

Dear Tom and Ray:

I called two Chevy dealers and one four-wheel-drive specialist and got three different answers. Then I talked with my boyfriend, who happens to be very mechanically inclined, and that was the icing on the cake of confusion. I own a '91 Chevy full size pick up with four wheel drive and need to know whether posi-traction is different from limited slip differential. I also need to know what I have on my truck. I got stuck in three and a half feet of snow! I have a bet with my boyfriend riding on the answer, so I hope you can clear it up.

RAY: You want US to clear this up, Jeannie??? This happens to be one of the most perplexing questions of the late twentieth century. Chevrolet alone has four full-time philosophers and a periodontist working on this issue as we speak.

TOM: Actually, Jeannie, "posi-traction" is just Chevrolet's name for limited slip differential on passenger cars.

RAY: Normally, on a rear wheel drive car, the power is delivered to both of the rear wheels equally. But because of the way the differential works, when one wheel starts to slip, ALL of the power is then delivered to THAT wheel. That doesn't help you much if that wheel is stuck on snow or ice.

TOM: To correct for that design problem, "limited slip differential" automatically transfers some of the power from the slipping wheel to the other wheel. That's why it helps you get off a patch or ice or snow.

RAY: On Chevy trucks, limited slip is called "locking differential." That's just a heavier duty version of "posi-traction," and it operates ONLY on the rear wheels, even if you have four-wheel-drive.

TOM: So what do you have on your truck, Jeannie? Well, we don't know. We know you don't have posi-traction, because that's only available on cars. You may have locking differential, but you haven't given us enough information to know that. The fact that you got stuck in three and a half feet of snow doesn't tell us anything. You're SUPPOSED to get stuck in three and half feet of snow, even if you have eight-wheel-drive.

RAY: So there are several other ways to find out how your truck is equipped. The easiest way is to check your original dealer invoice. If you've lost that (which we assume you have), you can check the equipment decal, which is pasted to the bottom of the glove box. If you see the code "G80" on the equipment decal, that tells you the truck has locking differential.

TOM: Or, if you want to do some "real world" experimentation, we suggest the Felipo Berrio test. Put the truck in your garage and put it in two-wheel-drive. Then pour a quart of Felipo Berrio Extra Virgin Olive Oil (make sure it's Extra Virgin) under the right rear wheel.

RAY: Next, have your boyfriend step on the gas. If the wheel just spins, you don't have locking differential. If, on the other hand, the truck shoots forward into the bicycles, the storm windows, and the old tires, then you do have locking differential. Congratulations!

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One