We had a 74 K20 and a 79 C30, they both ran 16.5's. Although they may have been options earlier, IIRC factory 16's became common in the 89 and 92 model years when the body style switched. Standard 4x4 tires were 245/75 R16's with 265/75R16's as an option (265...standard on Z71 package trucks). Only light duty (1500 and 2500 non C6P) 2WD trucks had 15 inch rims. Dunno if that'll help, hope it does.