Actually you fit a skinnier tire on a wider wheel to stretch the tire and make it stiffer so it corners quicker and transitions faster.
If you put a WIDE tire on a skinny wheel the steering will feel mushy and you will need alot of air to keep the tire from rolling over.
This of course relates to autocrossing.
I've seen some guys put 235 wide tires on 8-9" wide rims.
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