The differences in diameter between the two sizes is almost 2", so it would be a tight fit. You gain about 8/10" of rubber on the sidewall. But then again, a GT could stand to be lowered so it might fit OK on a non lowered GT. You would probably experience some rubbing on turns while going up or down driveways and such.
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Oh, and on the rear mat, I tried the one from my son's 06 Vibe in mine and it fit OK, except for the flap on the right side and it is about 2" longer, but that can be tucked down between the floor and rear seat backs. I'm planning to try and get one for a 05-08 Vibe with the Monsoon package since it is straight along the right side without the flap.