I have 9 inch wide 18" rims in the rear (245/40 -18 tires, to big). The fenders are not rolled and they don't rub, so with proper ET you shouldn't have any problem.
The M3's need higher ET so unfortunately I can't say either yay nor nay about ET 40.
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1994 Arcticsilber M3, 286hp, 18" AC Schnitzer wheels.