[e90yyc]

I recently built a little model in Excel to help me diagnose what I think might be a driveshaft vibration, which naturally led me to comparing engine RPM at various road speeds for the 6MT vs. AT. I don't know why, but I was shocked at how big of a difference there is!

I had never given it much thought before, but it's now obvious to me why BMW felt it necessary to have four final drive ratios for the N52 E90s. There's a solid 25% difference in engine speed in sixth between the AT and 6MT.

I've seen a number of posts where putting a 3.91 diff in is described as an upgrade, but seeing what kind of engine speeds I'm going to have at cruising speeds has got me a little concerned.

For anyone running the 3.91 with the 6MT: How have you found it? Worthwhile? Easy enough to live with?

I don't know why I'm asking considering I just installed a near-new 3.91 rear diff and have a replacement 3.91 front diff sitting on my bench waiting to go in, but hey... more information is always good, right?