dssur
+1y
you will potentially get worse economy with the steeper diff gear. I have a 98 318i and at 75mph it is turning almost 3600 rpm. It gets 32 mpg. I could swap in a numerically lower gear but all I would do is hurt acceleration and take the engine out of the power band at cruising speed. Consider what your end result should be, if its economy you would likely get more by duct taping up the body seams and switching to a narrower, harder compound tire than you would by changing the diff gear. Changing the diff gear makes sense for guys who swap in V8's.