Hi Steve, When our cars were new the tire companys used letters and they offered Gr70-15 which today is 225 70R 15.
About the largest tire that will fit in the wheel well is 255 60R 15.
These tires have an overall diameter that is the same as the old ones 27.1 inches. If you get a taller or smaller diameter, in will mess up your speedometer.
Thanks Pete, at the moment i m running a 255 60 15 on the rear so i guess my speedo reads correct.I m about to upsize to a 17" wheel so i ll have to find one that gives me the same diameter.Keep your vette as long as me,26 years and counting!