Lash apparently Dougal isn't following this topic. To get an answer to your question it may be more expedient to contact him via the Longstone Tyre website.
Jack, it is true the 15x10 Campy wheels came on the scene in late 1972 or 1973, but the 225mm & 285mm Pirelli P7s came on the scene in late 1977 or 1978. The 15x10 Campys where manufactured for a different tire which came on the scene in the summer of 1972 … the H60V15 Goodyear Arriva. It followed De Tomaso's convention of fitting tires on the widest wheel possible. The 15x10 wheel offset makes much better sense with a 255mm wide tire, which is set about 15mm inside the fender edge.
