http://www.tirebusiness.com/article/...-tire-industry
The big 3 in North America are Goodyear/Dunlop, Bridgestone/Firestone and Michelin, with over 50%. Conti is fourth with just 6%.
Goodyear is in Nascar and Firestone in IndyCar, so it makes sense for Michelin to become an IMSA partner.
However, I wouldn't be surprised if Dunlop or Bridgestone made a decent bid.