It's got a car engine... why not a car tire? It's not like the Boss Hoss is noted for its fine handling qualities, and all of that torque and HP eats up a m/c tire. All the disadvantages of a trike, and none of the advantages... is this what you buy when the midlife crisis hits and you can't afford a nice sports car?
I'm amazed at the folks who do things like this to a sport tourer (putting a car tire on a FJR), trying to save money, risking a lot more than money. Why would you do anything that would negatively affect the handling on your FJR? I know, the rationale being that the Darksiders aren't looking to lean through the turns with the best of 'em, but sometimes shit happens. Last week I was coming into an exit from one highway to another when a semi merged from an onramp onto my highway, so I hit the throttle to get around him and then entered the exit... only to find that it was much tighter than I thought. Engine braking, some front brake, and a lot of lean got me through. What would have happened if I had a car tire on the back? It would have been a lot hairier and scarier, to say the least.
I look at darksiding as akin to the fellows who ride their tires until the cords are completely exposed, just to save a little, or the private pilots who own their own planes and postpone oil changes and maintenance to save a little. Most of the time, you can get away with this, but eventually your number will come up. Hopefully, when that happens, you'll be able to kick yourself repeatedly (and not just mentally) while you recover.
The thrifty man spends the most. I value my life, and my health, enough to buy the right tires when I need them. If I need to buy them frequently, then that's the price for playing on a motorcycle.