3.91 Gear-set Review

I bought the differential from Performance Gearing and spoke with Jim Blanton, the owner. He is a great guy and knows differentials inside and out, literally. He specializes in BMW's, LSD's, and gear set changes. He typically sells made-to-order LSD's for about $3k, he also sells ring and pinion sets, etc. He just so happened to have a low-mileage 3.91 differential from a friend's X5, he had upgraded, which bolts up perfectly to the 6er (without his expertise I nor probably anyone else would have known this). I didn't mind staying with an open rear-end vs. LSD, so I opted to go that route.

Anyhow, I picked the car up Friday and have been waiting to give a review to get a good feel:

With the new 3.91 differential I gained just over 13% more acceleration and I can definitely feel it; with a nice, smooth, more powerful constant push against the seat at full throttle, better take-over power, and a little less MPG (I'm sure this is due to the gearing and in-part from the fact I kept "testing" it for the review").

All-in-all, to sum it up, the 3.91 gear set feels more "correct" for a country with no autobahn driving, although I have a good feeling the car would still easily hit the electrically limited speed of 155, this however I did not test.

Side note: Anyone looking to do a LSD needs to contact Jim and just speak with him, and they will be sold; he's the best of the best in BMW differentials.


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