Hi,
I was told by a friend I can go as high as 275/60/15 on the stock rims. So I figure 245/60/15 will fit easily in the back, assuming you havent' made any major changes to the suspension or anything.
Bye!
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Gautam N. Lad
http://www.cubicdesign.com
http://www.cubicdesign.com/mustang/