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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Great feedback JPEG23. I believe with the differential change and possibly doing the Dinan intake and software upgrade, is the most you can get out of the 6. Getting a 13% increase is a big step up in power as most after market add on do not come close in reality. This is something I would definitely do but unfortunately I have to cool my heels until my warranty ends on the car.

How do you know you've gained over 13% more acceleration? Did you measure before and after 0-60 times?

Originally Posted by JPEG23

although I have a good feeling the car would still easily hit the electrically limited speed of 155, this however I did not test.
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Does this lower the cars top speed?

Originally Posted by Tosh.645ci

How do you know you've gained over 13% more acceleration? Did you measure before and after 0-60 times?

Simple math, 3.91 (new gears) / 3.46 (OEM gears) = about 1.13006 and yes shorter gears lower the top speed. But with the car electronically limited at 155 and a true top speed of around 190; it will still hit max out with ease.

The gear ratio (ex: 3.91, 3.46) means how many times the drive shaft rotates for every turn of the wheels. So for every turn of the wheels my drive shaft turns 3.91 times instead of 3.46; in turn increasing the acceleration of the car, but lowering the speed at which the car switches gears.

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Great review. I would not mind doing this myself.

Finding a good used OEM diff with shorter gears is by far the best bang for the buck performance increase for any car.

Great job Jpeg, now we need a group buy! This sounds awesome.

Good stuff JPEG23...I might also change my gearing when I get my Quaife LSD. It's something I'm really going to look in to. Initially, were you looking to change your gears to a 3.91 or something else? If so, what?

Originally Posted by 2modfast

Good stuff JPEG23...I might also change my gearing when I get my Quaife LSD. It's something I'm really going to look in to. Initially, were you looking to change your gears to a 3.91 or something else? If so, what?

I researched what it looked like might fit on the 6er (I saw BMW had released a 4.11 on the X3's and a 3.91 on the X5) and then I e-mailed Jim and asked; he said the only OEM BMW differential with shorter gearing that would fit the 6er's was the 3.91 from the X5, so this made my decision pretty easy.


Before you go Quaife, I really recommend calling or e-mailing Jim Blanton at performancegearing.com and say JonPaul sent you (maybe he'll give some sort of discount). He really is the best in the business of LSD's on bimmers, they cost $3000 and his differentials just need lube and an installer, they also have a 3 year unlimited mileage warranty.

Install on a differential only takes about 3 or so hours. I got mine replaced for $231 (taxes and labor).

Jim really takes care of his clients and goes the extra mile to make sure you're happy. I was very, very impressed with the knowledge and service I was given!

PS - with the 3.91's I cruise at 65mph at around 2200rpm



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thanks for this thread this is all i been looking for.

JP is right, Jim knows his stuff. For the $3000 you get both the LSD and a custom gear set of 3:91 of even a 4:1.

I spoke with Jim at length about this. I asked him what the difference between his LSD and Quaife and he gave me a bunch of info about how his equipment works and the reason it is better. Don't have my email as I am travelling or I would pass that info on, best to just email him and get a dialog going. It is best by email than phone.

JPEG23...how's the diff. holding up? Any videos?

Originally Posted by 2modfast

JPEG23...how's the diff. holding up? Any videos?

He sold the 6er.

JPEG23 - Last Activity: 11-24-2012 06:49 PM

Yikes...he should have parted that thing out. I'm sure someone would have bought the diff. Oh well.

Im totally interested in doing this mod!