When you're looking for centre caps on ebay and the like, what measurement do you look for, overall diameter of the cap or diameter of hole in the wheel?

I had my calipers across the lugs in the existing centre caps and this should be the ID of the hole in the wheel but i can't find and centre caps that are 64mm or 65mm.

Could somebody, if it's not too my trouble - measure the ID of a Vauxhall wheel please, something 5 stud as i imagine it would be similar. Cheers