Differential problems.
 

I would match the front and rear diffs.

The 3.64's off the 4.4 can fit if you find used ones at a junkyard

Hi Ricky,

Thanks for reply. I probably should have mentioned that mine is a 3.0 diesel. Anyway I have found a matching front and rear 3.73 and hope to collect later today. As with your comment, I agree with the need to match them otherwise I imagine some nasty munching. I'm just hoping that I don't have any other problems regarding mounts, etc or ECU. Though I don't believe there are any sensors that would be affected by the change in overall final ratio. in other words It's not going to be any different than if I put on a smaller set of tyres/tires. I would just love to know if anyone else has done this...

I would actually imagine replacing with 3.73's together should not only improve fuel mileage but keep you at slightly lower rpm on the highway so a bit quieter as well.

Guys have replaced their 3.0 (petrol) diffs (4.10's) with the 3.64's off the 4.4 and noted nothing but positives. gGo for it!

That's good to know. Well at least I'm not a guinea pig. So a double benefit so. I also heard that the 3.73 diff is a much better one than the 3.91 as on mine. Better mileage would also be nice with the price of fuel. Thanks for the advice. I'm off this evening so to pick up the front and rear 3.73 diffs so. I'll report back with any details.

Lookin forward to updates!

No sure but I think the speedometer will be off. You will be going faster than it reads

Where does the x5 get it's speed from? Everything I have found comes back to the wheel speed/abs sensor. If that is the case, I don't think the speedometer will be off.

I agree. Speed is based on the right rear wheel sensor. Diff shouldn't matter. Changing tire size does matter...

That would be great, is that something you know for sure?

It's definitely driven off one rear wheel, which is all you are concerned about. This thread says left rear. I don't have time to get my Bentley out now to check...

http://www.xoutpost.com/bmw-sav-foru...ow-lights.html

Good to know-thanks.

Regarding the speedo sensor, it is on a rear wheel to my knowledge. On a previous 530D that I had it was on the right rear wheel. The speedo stopped working on it once and replacing that sensor solved the problem.

I can now also report that I'm half way through the swap out of the diffs. I picked up the 3.73 versions and all looks good so far. The rear seems to be very straight forward and so I have left to the end. However the front requires a little more work. I've only had a little time at it yesterday evening so far. Anyway so far I've had to pull out the front half-shafts by loosening the front wheel carriers. the shafts are out of the way the CV joint housing can be pulled out from the diff (there is a special tool to do this but I just used some ignorance to persuade its separation). The prop shaft to the diff was easily disconnected as too were the four diff mounting bolts. So solid progress was made in under an hour but then brought to a sudden halt due to the fact that the available space to remove the diff through is a little on the small side and will need for me to drop the K frame (which engine is mounted to) about 20mm. This should allow the diff to slide out from the side. BTW the K frame is held in place with bolts that require a female Torx socket to remove. Unfortunately, mine only go up to 16 and I reckon I need an 18 or a 20. I hope to pick one up later today and get back to it. So fingers crossed for the next hunt. I'll report back when I get further along.