I would go 185/60R15 on the rear. When you put smaller diameter tires on rear you have to adjust your stace. So you need to get lowering springs on the front.
I have 175/80R14 on the front and 175/70R14 at the rear. They are even different make. Last summer I had even 165/70R13 at the rear and had no problems with normal driving. My cars front is lowered 60mm and at the rear I have -30mm lowering springs.
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