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Aside from an increase in tire diameter and sidewall height, is there any noticeable difference between a 185/55 mounted on 15x7 vs 195/50 mounted on 15x7.5?

 
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If you are comparing the exact same tire brand and tread type, no. The smaller sidewall height should added rigidity in corners but probably not anything you would notice.

Now if one is an all season tire and the other is a performance summer tire then that is another story.