Has anyone tried to assess their market share in relation to their competitors in more detail than just global percentage? It is a tough thing to estimate, especially with all the legalities and inter-company resistance in place.

Toyota are apparently world leaders, but how do companies out there see who is strong in which countries and for what type of products? Do you rely on sales teams' rough estimates, or work directly with dealers? Does anyone know of any data/figures published anywhere which shows such information? Or alternatively, is this knowledge useful for improving the business at all?!