pulling a 735i differential

i know where i can get a good deal on a rear end from a 735i auto, i thought they had 3.64's but from searching it looks like it will have a 3.91. So that leaves me a few questions:

what would be the effects of going from a 3.15 to a 3.91, would sport mode be useless in 4th, would my speedometer be off(if so by how much estimated,) would i get better city mpg, or would i get worse with both hwy and city?

my other set of questions is, how common are these diffs. LSD? with either the 3.64 or 3.91? If the 735 and my 530 is auto it should be a direct swap correct?

just looking for some info before i pursue this, the car is a bit of a drive to get to, and i have no way of having the case looked at for the stamp/serial.

im keeping my fingers crossed that its a 3.64 lsd, i can have this diff. for $100 pulled.

thanks guys

sorry, i believe it is a 89 e32 735i

anyone wanna chime in? should i have this moved to the 7 series section? Shogun, Gale?

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Originally Posted by Boone.Msi

anyone wanna chime in? should i have this moved to the 7 series section? Shogun, Gale?

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I can't offer any knowledge in regard to performance with the higher ratio differential, but I do think that your speedometer should still be correct with the new differential since it is basically driven by a magnetic pickup (or hall-effect?) sensor. The pulse should not be any different between the differentials. Just my $.02

the speedo should stay correct as it measures axle rotation and not driveshaft rotation, the only snag would be if the input flange is different on the diff, don't get too many 530s so i can't remember what size flange they run but theres a couple choices on the e32 depending on production month, they can usually be swapped but if you overtighten it it can destroy the pinion bearing on the new diff in short order

Post '89 US spec E32s have 3.91 gearing until '91. The gearing is all wrong for an automatic M60 530, your 2nd gear will become what 1st was, cruising RPMs will be much higher. The difference is around 25%.

I'm not anything like an expert on these things, but I think a 3.64 will be too short as well. You want a 3.45 or 3.46

Speedo accuracy has to do with tire ratios. You don't have to replace a speedo gear when changing driveline components in these cars :)

$100 is a good deal if it's a limited slip, regardless of the ratio.

The 3.64:1 lsd is a great diffy for the 535 5 speed.

I put a 4.27 lsd in my 89 535. It had a stock 3.91 in it. The 4.27 was from a 735 and the input flange was a direct fit. I have more power in town and more noise on the highway. More noise and more gas use then I thought I would have. I love the difference the lsd makes especially in snow here in the midwest. I would not put in any other kind. I may go back to a 3.91 if I can find one with lsd cheap enough.