wheel size question

[i]Does anyone know if you can run a 15 inch wheel on a 1800 vtx I used to run 15 inch volkswagon tires on my old 650 triumph it looked cool and rode well I am just wondering if they make a 15 inch wheel for a 1800 vtx if it would work you could run car tires on front and back.it has been about 30 years ago since i was riding the old triumphs and I can,t remember but I think the tires were biosply also has anyone ever ran a syped tire on the back of these bigger bikes I am thinking of trying an all season radial syped tire I think it probably wont wear as long but it may just mimick a motorcycle tire.Anybody know of a reason not to run the syped tire?? Might just work well as a motorcycle tire is softer I think a syped tire will work