Its time to put a new rear tire on the drifter. What is the widest rear tire you have been able to fit without any issues? Also what brands have you been happiest with? This is the first time replacing a tire on this bike.
This might help. I run metzlers, have used several different brands and prefer the metz on the drifter.... I use dunlops on the goldwing but didn't like them on the drifter.
http://www.vulcandrifterriders.com/mods/metzeler.pdf (http://www.vulcandrifterriders.com/mods/metzeler.pdf)