Just put a 165/15 car tire on my 04 Montauk. Much softer ride. Feels like a 4" pad under your butt. Handeling is equal to or better. Had to re-adjust the lean out to a little less lean out, and now the rig is completely neutral. Hands off at constant speed (thanks to a throttle lock) the rig runs straight and true. Steer it with your head leaning from one side to the other. Really amazing. Tugging a Champion Escort and actually achieved the highest speed ever today prior to adjustment. Only have about 100 miles on tire, but trying to fix that this weekend. If it will just quite raining.
Uber
OK you guys have got my head all messed up. That does not take much some times. I think I read some where the only wheel you could NOT put a auto tire on was a 15”. Now Jim just did it. help.
The motorcycle rim can be modified to accept a 15" car tire, or a wire spoke M/C hub can be laced to a car rim to make it safe, but it is not a good idea to simply stretch the tire to make it fit.
There was just a huge discussion about this over on SCT. According to the Tire & Rim Association, the governing body for wheel and tire sizes on everything from Bicycles and wheel barrows up to earthmovers, the specs are the same for all but the 15" rim
Motorcycle: 
Auto:
Putting a 15" car tire on a motorcycle rim requires stretching the bead in excess of 1/4" beyond it's normal breaking point. Even if you don't see damage, it could easily affect the wire bonding and cause a catastrophic failure down the road.
Rather than repost the entire discussion, go to http://autos.groups.yahoo.com/group/SCT/ and search for the following subject line***
Re: w/ 15" rims added - Dicussion Car rear tire - Rim specs - MC & Auto
Put in a tube? That's the only thing I've ever done. Tubeless tires don't usually seal with all them little spoke holes. :0)
