I posted a while ago in the intro thread, and was directed this way because duh. XJS question, go to XJS forum.

So, right in. I have the wrong diff... Maybe. The PO of my car kept meticulous service records, so I know the diff has been swapped once before. However, the thing is a total DOG off the line (like there's literally no difference in acceleration between 1/2 and full throttle until about 4k), and the speedometer is off by ~20%. I physically counted the rotations in the diff, and it looked to be about 2.75:1, so I set about finding what I thought was the appropriate diff, a 3.54 Power-Lok. This is from the parts listings of my VIN in the part catalogues at both SNG Barret and Coventry West, but when I got a used diff from Welsh Enterprises... It measures about 3.0:1 (I assume this a 2.88, I'm measuring by eye)!

So, final consensus from actual knowledgeable people, WHAT is the factory final drive ratio for a 1993 XJS 4.0 Auto? 2.88 or 3.54?

 

Your car should have the low bias power-lock 3:54 differential ratio because it has a 4 speed ZF transmission with overdrive.

You 2.75 count and the dog off the line = 2.88. The lower gearing and 4th gear allow the 4.0L to deliver adequate performance in the relatively heavy XJS.