Originally Posted by Joshdub

Well some AWD systems can be very picky (X-drive being one of them), and 2% is a pretty good difference. I think we can deduce the answer from this thread alone really. You said it works fine on your 18's, so unless you busted a sensor or something while swapping them out, it's likely the difference in rolling diameter. Your pre-x-drive X5 was probably fine because the tcase was rather primitive compared to the newer X-drive unit.