see here for some dimensions, the diameter where the clips meet the caps is about 1/2" less than where you're measuring - or you could scare up a cap you can compare to your wheels .... AFAIK the cap dimensions are the same across years for the 15 & 16" wheels, and between cars and trucks, seems reasonable that the 5 lug car wheels would only have 5 clips, but the car caps still fit the 3 or 6 clip trucks rims
I'd think w/ 5 holes that there wouldn't be 3 evenly spaced so using all 5 would keep them more secure, don't wanna hafta go scouring the ditches to retrieve them!
Bill
