I have a roadster with the factory spare and the tire is indeed a 145/70/18 even though i have the sport package.
One concern I have though is putting the 18 inch spare on the rear. That can't be good for the diff unless they are the same overall diameter. With a flat on the rear of my S2000, which was also one inch undersized, the manual wanted you to swap a front wheel to the back, while putting the spare on the front. So the spare was probably not the same overall diameter.
Last edited by Ubetit; 10-03-2012 at 10:31 PM.
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