I'm pretty sure it's 8 inches wide. There has been a lot of debate over the widest tire that can fit on an 8 inch wide wheel...most say that 245 is the way to go, maybe 255 being the widest. Some say they fit 275 without a problem, but most say that is unsafe due to the tire bulging over the sides. I've never heard of a 285 on an 8 inch wide wheel, it would probably be very unsafe, even if it was able to fit.
275s look sweet, and my tire shop said they'd be fine on the 8". They handled pretty good, but don't expect any sidewall flex at all, if you decide to try and drop the pressure for some drag launches.