assuming you're just putting higher aspect ratio tires on the stock rims, every bit you go up in aspect ratio should reduce your rpm by a matching amount (equivalent to the increase in overall diameter of the tire), in your case, about 3%
185/60/15 (185X0.6) = 111
111X2 = 222 / 25.4 = 8.74 + 15 = 23.74" diameter
185/65/15 = 120.25
120.25x2 = 240.5/25.4 = 24.46" (3% wider = 3% reduction in RPM, and 3% error on speedometer)
185/70/15 = 25.19" (3% wider than '65')
185/75/15 (if that's even a size) = 25.92" (2.9% wider than '70')
So to drop 300rpm from 3000, you'd need to go from whatever you're at, up 15% on the aspect ratio.
As far as fitting inside the wheel arch, you should be fine, prior to chopping up my car I had 205/45/17's (24.26"D) on, and the only reason they rubbed a bit was because instead of being on a 5.5" rim (stock), they were on an 8.5" rim.
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