The Thunderbird is designed for performance handling. Common sense should dictate that the shape of a car tire, and the harder compound will compromise this. Sure you can ride around on a car tire, but doesn't it seem rather stupid to have spent the extra money for a performance bike only to then sacrifice that performace to save a few bucks on tires?
What if the owner of the said bike bought it to be a cruiser and not a performance bike? Is he still stupid if he decides to run a car tire? I thought the Tbird was a cruiser, not a performance bike. If you want to scrape pegs in turns should you have not bought a Triple Speed?