Quote:
Originally Posted by Armin_kh
I thought as long as the difference between front and rear tires is 30mm on Xdrives it's fine. I was running on 255/285 on 373 and I took them off after 3 weeks. It damaged the transmission. So you think if I change the 288 to 295 it will be fixed?
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A 295/30/20 tire is 27" tall. So is a 255/35/20, which is why they are a perfect match.
A 285/30/20 is only 26.7" tall, so you are starting out with a difference of just over 1% when the tires are brand new.
Tread depth is 10mm. At half wear on the rears, the tire will only be 26.3-26.4" tall. If the front are wearing half as fast at the rears, the front tire is now 26.8-26.9" tall. The difference is around half an inch, which is approximately 2%.
The generally accepted limits in tire height difference for the x drive system is 1%. But this applies at every point during the life cycle of the tires. So if you start out with a 1% difference, you need to know what will happen to that percentage over time. On the 6 series, the height difference will increase over time because the rears wear faster than the front, and on the setup you are using the rear is already the smaller tire.
To answer your question, a 30 mm difference is roughly 4% on these cars, which is 4x what BMW recommends, and would be very bad for your X drive system. Switching your rears from 285/30 to 295/30 won't reverse any damage you have already done, but it will work fine on your car and will not cause any further damage. I've run through 3 sets of rears and one set of fronts on my car with 255/35 front and 295/30 rear with no issues.