My mom and i both picked up e90s about a month ago, Mine is a 6 speed 330i and hers is a 330Xi Automatic. I did a diff swap in my 330Ci and loved it. I am thinking about taking my moms car for the day and swapping My 3.15 into her car and taking her 3.64 and putting it to my car. I wonder how much slower her car would be with a 3.15... at lease she will get better gas mileage. Will this affect the X drive taking her diff or no? anyone?