Im looking for a tire calculator or formula that shows for every inch in rim width, you can go a certain width in tire size.

For an example, a 17x9 rim will allow you to put on a wider tire safely versus a 17x7 rim.

I need this because I picked up a set of 17x9 Rota's and I want to put a wide tire on that isnt stretched. I hate the look of stretched tires. I know the rims and tires will stick out a little past the fender, but thats ok since Im having a body shop mold on a set of fender flares to accentuate the rims sticking out some.

 

I think this thing is still going strong after 10+ years:
http://www.miata.net/garage/tirecalc.html

as far as tire widths, that's going to be difficult. Search Google/forums to see pictures of tire widths with rim widths. The current fad is to stretch a tire, so even if they're running 245 it could be barely covering the rim.

edit: sorry, just realized the calculator doesn't incorporate rim width.

http://www.tyrestretch.com/

215/35/17 on a 17x9: http://www.tyrestretch.com/9_215_35_R17/
215/40 http://www.tyrestretch.com/9_215_40_R17/
215/45 http://www.tyrestretch.com/9_215_45_R17/

yuck... make sure you go bigger than 215/35. I'd look for something that shows 245/40 or 275/40

 

Ideal width for a 17x9 is 265. Between 255 and 275 will look fine.