hm... i jut learned how to do this yesterday so you may want to wait for more replies...
the GT's 15" alloys are 195/60R15 which has a final diameter orf ~24.21in, the 215/40R17 you have spec'ed has diameter or ~23.77
divide 23.77/24.21 = ~0.9818, and from what my informed friend told me you are safe under 2% so yeah this combo should work fine and won't screw up your suspension settings... by my calculation at least...
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