Sam -
I blew up some close-ups I have of these, and, best I can tell, it looks like 245/45x18 on front, and 285/40x19 on the rear. Please bear in mind these wheels AND tires are prototypes custom made for the GT Concepts models - the exact same tire may never have made it to production. I've seen a lot of really sharp wheels and tires over the years in the Ford Design Center that never made it into production.
Jeff