Really? The car's DSC is sensitive to a 3mm change in diameter? Wow.
3mm is .118", or roughly 1/8th (4/32nd) worth of tread depth. EVEN on a car that has squared tire sizes (same tires front and rear), in daily operation it is likely that the difference in tread wear will quickly exceed 1/8th of an inch difference between front and back on RWD platform. 1/8th inch in tire wear is about 1/3rd of typical tire tread depth (Conti DWS tread depth is 10/32")
Plus, if you could just humor me, the outer diameter of the 18" OEM wheel package and the 19" OEM wheel packages far exceeds 3mm. In fact, the front tire diameter of the 19" OEM wheel is .17", or 4.1mm larger than the rear. Are you guys suggesting that ALL Z4s with 19" OEM factory wheels have different DSC coding that eliminates that problem with different diameter tires?
This is the first I heard of this. Where is the technical documentation that says you can't have a diameter difference of more than 3mm?
There's a SIMPLER explanation than the DSC on the car is so sensitive that it can sense a 3mm difference in diameter. And that explanation is new tires come with MOLD RELEASE. As the tires are formed and molten rubber compound is injected in layers into a mold to shape the tread of the tire, each mold is coated with a thick layer of mold release to allow the rubber to separate from the mold as it's cracked open, otherwise hot, sticky rubber will stick to the mold and you can't form the tread pattern. that mold release is stuck on the top layer of tread, and brand stinking new tires can take between 200-500 miles to wear off that mold release off of the top layer of the rubber. Before that happens, all new tires tend to behave differently than their more worn-in brethren if you choose to only change 1 or 2 tires vs. 4.
If the DSC or the chassis dynamic is really sensitive to a 3mm overall diameter change, then you can't change from 18" to 19" wheels and vice versa.
By the way, as far as *I* know almost ALL BMW DSC/DTC/MDM/whatever chassis dynamic traction system allows for up to 3% difference in rotational speed differential from front to rear. On a 25-ish inch diameter (I say "-ish," because stock, OEM diameter can be as small as 25.02", and as big as 25.20") that allows for almost 0.75" of difference in diameter front to back.
All except for the E89 Z4, I suppose.