a friend of mine has a BMW r1200rt. No one in the BMW world talks about running a car tire like us Valk riders do. Is there any reason why he couldn't run a car tire. If he could, would anyone know the size?
I think one of the major reasons is the size of the rear wheel. The R1200RT has a 17" rear wheel. There aren't too many car tires in 17". Let alone high performance radials. That will fit in the space designed for a 180/55/17.
Valks, and a few others, use a 16" wheel and gives you lots more choices. Most 17" rims are designed for truck or pick-ups and 225 is considered a narrow tire.