[Pyrat 2]

I am interested to know where the sensor is for detecting the speed of the car. The reason I'm asking is I want to understand what will happen if I change the rear end differential gear ratio to a 3.38 or 3.46. If the speedometer sensor is off of the abs system I would expect that there would be no speedometer error. If on the other hand the sensor works off of the driveshaft then I can expect an error in the speedometer reading.

[volinder]

Originally Posted by mdemange

I am interested to know where the sensor is for detecting the speed of the car. The reason I'm asking is I want to understand what will happen if I change the rear end differential gear ratio to a 3.38 or 3.46. If the speedometer sensor is off of the abs system I would expect that there would be no speedometer error. If on the other hand the sensor works off of the driveshaft then I can expect an error in the speedometer reading.

Wow, that sounds like a really inexpensive mod!!!

Just outta curiousity, why would you want to do that on your BMW.

[Pyrat 2]

It is pricey unfortunately - about $2K. The higher gear ratio gives a nice performance boost in acceleration. Additionally, if you get the higher ratio with limited slip you get the added benefits of this if you autocross, track the car or drive in snow. Since the standard differential is open the traction control can only limit the wheel spin on the one drive wheel.

[MMira]

could you please slightly elaborate on what are you going to do then?

only LSD or something more?

[volinder]

I see. I can definately see the advantage of the limited slip differential. Just seems like some big bucks, not to mention the loss of warranty on the some vital parts of the car.

[Pyrat 2]

Cost is about the same as a set of wheels and tires and gives a definate performance boost.

[Pyrat 2]

Anyone know the answer to my question?

[akhbhaat]

Originally Posted by mdemange

Anyone know the answer to my question?

Changing the rear end ratio should not have any effect on speedometer calibration. This is, at least, the case on the E46 and should follow likewise for the E90.

[akhbhaat]

Originally Posted by volinder

I see. I can definately see the advantage of the limited slip differential. Just seems like some big bucks, not to mention the loss of warranty on the some vital parts of the car.

Vital parts...such as? Aftermarket differential shouldn't affect any warranty on any part save the differential itself...and a good differential will carry its own seperate warranty. No worries.

It's entirely possible to pick up a slightly used OEM open differential from a wrecked car for a relatively small sum - $500 at most. The 3.46 swap (which is the rear end ratio used by automatic 325s) is a very popular modification for drivers of manual E46 cars. You won't gain the desirable limited slip effect, of course, but you will still experience greater torque at the wheels in each gear and it's an easy direct swap and very inexpensive. I'm sure, sooner or later, this trend will pick up in the E90 community as the design ages and becomes more widespread and the availability of parts increases.

However, I've noticed that the E90 uses a very aggressive rear end in the automatic cars - even more aggressive than that of the E46 M3. No doubt this has something to do with the move to a six speed gearbox. This mod might not be so popular in the E90 community, after all...

[Pyrat 2]

The 330 e90 manual transmission car comes with a 3.15 ratio and the auto comes with a 3.64. Anyone know if the e90 rear diff is the same as the e46. The manual transmission on the 330 e90 is the same as the e46 performance package 330 so I was just wondering if the diff was the same.