Originally Posted by oldskewel

I have a 3.0i, with 19" wheels - 285/45R19 107V rear; 255/50R19 103V front; 235/65R17 spare

I once calculated the rolling diameters of these and found that they are extremely close, which makes sense since BMW designed it that way.

To keep your differential and transfer case happy, you just need the rolling diameter (radius, circumference, etc.) close enough.

Quick calculations on your tire size vs. my 17" spare shows them 1.3% apart (your 20 is actually the smaller of the two. My 19's are even closer than that.

radiuses calculated here in inches:
17/2 + 235*0.65/25.4 = 14.51
19/2 + 285*0.45/25.4 = 14.55
20/2 + 275*0.40/25.4 = 14.33


So if a 17" spare will clear your brakes, it should not cause problems with the diff and TC.