G'day Lofty & welcome from Hervey Bay Qld mate, as you've gone through 5 already, when you're pissing no 6 off there's plenty here that would accept it as a gift.
Granny D, the topic you have broached is a real can of worms. Suffice to say that there are plenty on here who use car tyres on their Rockets for a whole range of reasons from economics to improved ride and handling to better grip and traction. If you're lucky the discussion will be left at that. You could do a search for "darkside" and see some of the views and also how passionate some members are about their choice. For me, I'm totally cool with whatever tyre choice anyone makes and I don't believe they need to justify it to anyone or get too caught up with defending it. But that's me.
FYI, I run a Bridgestone Exedra Max 240/55x16 motorcycle tyre on my motorcycle.