first off a wider rim means you run a wider tire, if you would plan to use the same size tire going from a smaller rim to a wider rim... that makes zero since as whats the point? Plus it looks stupid to have a tire stretched out on a rim thats to big.
The center number in the tire (275/40/17) the 40 is the percent of the 275 so 275 * .40 = 110
That 100 is millimeters (mm), thats how tall the sidewall is
275mm is how wide the tread is, 110mm is how tall the sidwall is, and 17 is the rim. So you need to maintain the sidewall height from stock so the speedometer is right
if you go up and down in rim sizes the sidewall has to go up and down to accomodate.
1 inch = 25.4mm for conversion needs
Sizes for 94-04 Mustangs
17x8 needs a 245/45/17 or 255/45/17
17x9 needs a 275/40/17
17x9.5 needs a 275/40/17 or 285/40/17
17x10 needs a 295/40/17 or 305/35/17
17x10.5 needs a 305/35/17 or 315/35/17
its not a perfect measure but you try to get as close and keep a wide enough tire to protect the rim lip without being to narrow or two wide.
With all that said there is no actual standard for tires.... some tire brands run wider/narrower than others.... have to find pics and research it to be 100% sure. Most are rather accurate.