I would probably stay with a
motorcycle tire for the rear if it wasn't for the greatly reduced longevity. From my past experience I get at least 5 times the miles on the car tire. I really don't know how long the car tire will last because I have never gotten close to wearing one out. On my VTX 1800 I had 3
motorcycle tires; 1 stock Dunlop, 1 Dunlop Elite 3, and 1 Avon Venom, all lasting from 5 to 6 thousand miles till steel cord was showing. I put a 205/65/16 Michelien Hydroedge car tire on the bike and road it for 20 thousand miles, it looked like it had at least another 20 thousand left on it when I sold the bike.
There were things I liked better about the car tire and some not. I liked of course the longevity but also liked that with the car tire I could go 15% faster in a curve and still feel in control of the bike, the ride was smoother over bumps, and bike had much better traction; these are some of the positives I can think of. The only real negative to the car tire was that on uneven roads at slow speed the bike would rock, which for me was not much of a problem. On my
1800 Goldwing I put a General tire which did not handle nearly as well as the Michelein Hydroedge, it felt mushy even with max air pressure. I had a Michelein Hydroedge on my Stratoliner but the only size offered with the Hydroedge close to the sized I needed was a 215/60/17 which was to big. The tire handled great on good road but bottomed out on rough road, also the larger width made the bike not handle quite as well as the VTX.