Stock tires on an 01 GT are 245/45/17, correct? If so, the overall diameter of the wheel+tire is:
(245mm*0.45 = 110.25mm) + 17" (or 431.8mm) = 542.05mm = 21.34"
and for the 275/35/18:
(275mm*0.35 = 96.25mm) + 18" (or 457.2mm) = 553.45mm = 21.79"
Pretty close, but ideally you should...