its the type of tire. a 245 tire in x brand may not be the same as another 245 tire in y brand. tires differ greatly from brand to brand and more so with slick/race tires.
case in point: i am runnin 255/40/18 on my 06 sti with cobb springs and it fits just fine.
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09 Pontiac G8 GT
08 S2000 CR (sold)
06 Sti (sold)
98 ITR (sold)
88 civic 4 door (sold)
95 Mustang GT (sold)
87 LX 5.0 stang (sold)
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