I would put wheel/tires on car and use tape measure to measure from outer tire to inner lip. Leave yourself a little clearance to avoid rubbing. You can drive car on a incline (i.e. driveway) diagonally and do measurements there.
Akra GT4 / MS Stage 2 / MS UDP / BPM Tune / KW CS / H&R Sways / TMS Delrin / ST 380/355 BBK / Cantrell 2.5" Ducts / SR34 Pads / Apex Arc-8 / Hoosier R7's / Mode GT4 Splitter / GT4 Canards / APR GT-250 Wing / Cantrell Half Cage / Recaro Pro Racer / Schroth Profi II-6 / BMW Alcantera Wheel / Cool Shirt / ESS Fire Supp / Solo DL