Just a few of my experiences on bigger and smaller tyres. It is normally fine to go up or down *ONE* size from manufacturers specs (depending on bike and wheel type). Turn in speed will be slower with a bigger tyre, but grip is better. A new tyre always has better turn in cos the profile hasnt squared off yet (unavoidable in commuting). On the front, a size smaller gives you better steering feel. I have done both of these on both the CB125 and the MZ. Both I was happy with the result.