Well first off, the width of your wheels is what we need to know, not the diameter. Second, I don't know who in their right mind would put 215s on a mustang haha. Guess you should look at other local tires shop that may be a bit less crazy lol.
2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts