Good data:
http://www.tirerack.com/tires/tiretech/tirespecskey.jsp
"A tire mounted on a narrow rim would be "narrower" than if the same size tire was mounted on a wide rim...Because of the different wheel widths used in the above example, there is a 8/10" projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. "
http://www.dunloptires.com/care/widthRatio.html
"Correct rim width ensures flex at the designed flex point in a tire sidewall for optimum tire performance.
If the rim is too narrow, the flex point moves toward the shoulder area, creating heat buildup in the shoulder, which reduces tire life and could result in failure."
http://www.yokohamatire.com/utrimwidth.asp
http://www.yokohamatire.com/utwheels.asp
"Choosing a wider rim: increases vehicular stability, steering response and cornering ability. A rule of thumb is to use a rim width 90% as wide as the tread width (not section width) of a performance tire for street applications. This provides a good balance between performance and ride quality. Always be sure that the chosen rim width is within the tire's range of acceptable rim width specifications."
So many Mustang owners put oversized tires on their rims and say it handles better. It a delusional claim. I have done it and ended up regretting it. It's a sloppier ride, no ifs ands or buts. There are no performance street cars that have such a disparity between rim and tire widths and there is a real, proven reason for it. That's why rims are available in different widths. A smart tire shop won't mount a 295/305/315 on a 9" rim because it's a liability.
Oversized tires is right along the lines of a huge aluminum wing on a stock Civic - you may think it looks good but don't claim any performance benefit becuase it's strictly ornamental, and in the end, only hurts all-around performance.
