Ok, I continued my searches and reading and actually found some interesting threads.
http://www.chiefdelphi.com/forums/sh...traction+width was my starting point. It is quite a long thread discussing Friction as a function of area. This lead to a pair of white papers
http://www.chiefdelphi.com/media/papers/1381 and
http://www.chiefdelphi.com/media/papers/1382 which have some interesting conclusions.
I can keep searching or you could wait until you get into the shop and do some tests yourself. Whichever.