According to Tire Rack, the widest they recommend on an 18"x8.5" wheel is 255/45. BFG has that size for the KDW-2. Section width is 10.1", No measurement for Tread Width(other tires that size are ranging from 8.7-9.2"), Overall Diamenter is 27".
The ones you have now-Section width is 9.6", No measurement for Tread Width(other tires that size are ranging from 8.1-8.7"), Overall Diameter is 27.2".
Agreed, but it depends on the brand of tire. Nitto's run narrower than other brands of similar size. I'd bet you could put a Nitto 555 285/40/18 on an 8.5" wheel safely.