From what i understand it is the width of the tire which makes it rub.
The stock non-sport 16" wheel has 47mm offset and with a tire width of 8.6"(205 55 16) there is a gap of about 1.25" from the fender.
The M3 rear has 20mm offset which is 2.7 cm outwards from the stock coupe wheel (47 - 20 = 27mm/10= 2.7 cm). If the tire width was to have remained at 8.6" it would have been ok.
The tire width is i believe 10.2" (255 35 18), a gain of (10.2 - 8.6= 1.6" over the stock non-sport width. Divide it by 2 , we arrive at :
a.) 0.8" increase at either side of the wheel.
We now convert 27mm to inches, giving us 1.063". We cannot divide this by 2 since offset occurs only towards one side, in our case towards the fender.
We add the increase in tire width 0.8" (from eqn. a) to the offset 1.063" giving us 1.863" increase towards the fender.
Thus the M3 rear wheel will not fit the coupe, if lowered or no camber adjustment.
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