Yep. The offset for the older rims is typically +30mm (believe its +24ish for the stock rims) or below, where the GP needs rims between 35-45ish mm. Fortunately, the newer Mustangs have that offset.
If I remember correctly, the new Mustangs have 45mm offset.
The bolt pattern is fine. GPs have a 5x115 pattern, and the 5x114.3 on Mustangs works fine, it's just the offset that ruins it.
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Originally posted by HercMan:
Sounds like corrosion has had it's way with your nuts.