I'm sure if I cruised at your cruising speeds, mine would go down to the same. I'm still heavily considering trying out the 50i diff swap, which would lower the final drive from 3.46 to 3.15. If I remember correctly when I did the math, this would make the new 5th gear 5% shorter than our current 6th gear, so at worst case, towing would be done in 5th instead of 6th. For every day use, I'd love to have the longer final drive, but the car is geared phenomenally right now for towing. 95% of my driving is just highway commuting, in which I would absolutely love to see the 3-5 mpg gain that the 8 speed in the f15's get on the highway. I love the 27mpg I get with a heavy foot, and the 28.5-29.5 I get wiht a light one, but netting 32-35mpg on pure highway would be really beneficial for me over the next few years. I'm well on track to run about 40k miles a year, so it wouldnt take long to recoup the $3-500 in parts I'd spend on the differentials.

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'10 X5 35d- 279k mi, deleted and DUDMD stage 2, daily and tow vehicle
'01 330i- Drift car: SLR Super angle, BC coils- keep it simple
'07 335i 6mt - Sold @ 171k miles- stage 2 daily commuter for 2 years