1: Shorter sidewall, so less flex, so crisper handling.
2: You reduce the percentage height of the wheels. If you have 225/50-16 now, you use a tire size calculator to find the comparable size for 255.
Here's a calculator. The best size for a 255 would be 255/40-17.
3: Tire width depends on the width of the new wheels you choose. It also depends on what you can fit under your fenders, even after curling the lips.
Good luck.
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Jack Olsen
1973 911 T sunroof coupe
[This message has been edited by JackOlsen (edited 10-16-2000).]