Quote:
Originally Posted by baron95
In the case of the G80/82/87 there are BMW interviews and article/quotes explaining their rationale.
Basically they fit the largest rear tire that still meets the fuel economy and unsprung weight parameters. Then, the fronts have the same rolling diameter, but with taller sidewall to minimize bump/impact harshness. It is as simple as that. 275/35/19 on 9.5" wide wheel and 285/30/20 on 10.5" wheel have the same rolling diameter, but the 35 series in front absorbs impacts better, while the 30 series rear provides less sidewall deformation and therefore more consistent traction into/out of corners, which also makes it easier to program traction control.
No need to overthink.
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275/40/19 F and 285/40/19 R for F90 M5
245/40/18 F and 255/40/18 R for M440i