I say that if you find a car tire the right width for that 250 rim, the height profile (50 series, 40 series, 35 series) won't be right to fit under the fender, especially on the side corners of the tire, the fender wraps to close to our rounded tires.
Import bikes like the VTX1800 (my father has one and I researched the car tire thing for him) have a ton of fender clearance that our bikes do not have.
I would say that IF you find a 35 or 40 series 18" tire that will fit under that fender (which you won't) it WILL handle like dog shit... NO, it won't handle at all.
Here is a test, let ALL the air out of your rear tire so that the bike sits on the sidewalls... Now go test ride it, that is how it will handle!!! It will be like riding your bike now WITH NO tire on the rear of it!
Good luck and I can't wait to see how your venture pans out, just for the info!:2thumbs:
