You want the rear to match as close as possible to keep the differential happy. A pull to the right or left is easy to manage for a few miles, a burned up m diff is a different story and I want a spare I can drive on for hundreds of miles if needed. The viscous lock/unlock mechanism in the diff is easy to overheat on a racetrack with same size rear tires, imagine a 3% difference in wheel speed for a few hundred miles
The rear size will rub on the front fender liner under certain conditions
A nineteen is big enough to fit the BMW jack kit inside and an eighteen isn't fyi
Last edited by Richbot; 10-30-2011 at 08:03 PM.