I had some service work done on my XJS this week. Power steering fluid flush and refill, flush and refill differential, 4 wheel alignment, and rotate and balance wheels. I asked them while they were under my car to find the differential ratio. They came back to me with 3.54 to 1. That is higher than I thought it would be. Does that sound right to you? What is your ratio and would you change it? I was thinking it would have been 3.00 or 2.80 for a grand touring car.

 

The V12s have 2.88 posi. The 6 cylinder has 3.54 limited slip

 

according to the paper work they gave me, the rear axle is 0000000588176 power loc 3.53:1. It does lock and unlock when in a slow turn left or right with on throttle applied. I don't think a limited slip does that or does it? The rear end action is the main reason I had it in there. They changed the differential oil 4 years before (10,000 miles ago) when I purchased the car, along with the brake fluid and coolant. shortly after that is when I noticed lock/unlocking action. I remembered years ago my new 67 390 GT Mustang with a "traction Lock" rear had to have an additive in the differential oil for locking rears. Just for curiosity I looked up my paperwork on my jag work back then to see if I was charged for an oil additive. No additive on my bill, so I had Jag look into that and they said the additives had been incorporated in differential oils and nothing else was required. I am not sure if there is "snake oil" in all this additive stuff...

 

I'm no great authority, but I believe the sixes got 3.54:1 where the V12s got 2.88:1 (or 3.31:1 or 3.07:1 pre '82.) If I were going after my transmission, I'd probably do something about the final drive as well. I may or may not have options based on what I've read on the board, and in the Book.

As far as the limited slip additive, I still smell it in my nightmares sometimes. I don't know if it still is a real thing, but it sure was.

 

In very broad terms, the cars post 1982 with 3 speed transmissions have 2.88 rear axles and those with 4 speed automatic/overdrive transmissions have 3.54 axles.

Powr Lok is a trademark, thus the spelling. The axle is always locked, unless it needs to unlock, such as when turning a corner. It is quite different from the traditional US style limited slip axles. Although both Ford and Chrysler have used the Powr Lok in the past. It's a more complicated axle than the typical Ford/GM limited slip, which is why it's probably not used by them any more. Among the Chrysler big block owners, it's widely regarded as the strongest type of limited slip available.

The 2.88 was a compromise brought about by the need for better fuel economy for the V12 cars and still working with the 3 speed transmissions, as there wasn't an automatic with overdrive that could handle the V12's power in the 1980's. The V12 at the time was among one of the most powerful production engines produced in the 80's, it did roughly 100hp more than either the Ford 460 or GM 454.