In my motorsport days, I've had both 175's and 205's on 6 inch rims. The 175's sit very square shouldered to the rim, while the 205's bulge out. If I was dictated by tyres, I'd say the 205's should be on 7 or 8 inch rims. If you look closely, you'll be surprised to find the difference in contact patches on the road is VERY small if both on 6 inch rims, as the sitting square on the rim allows a square contact patch, if that makes sense.