We run a fleet policy, and I have had this conversation regarding "C" tyres, they didn't give a monkeys. So you can have your own conversation with your own insurer regarding this matter. As I understand it, the C tyre is a tyre that is designed for the fleet market, i.e it lasts along time. In my experience that is it, pretty rubbish in every other aspect. A 106xl tyre with all season designation is far far better than any rubbish "C" tyre.