From reading posts about what I'm going to do with my 10" wide rims when I get them, 295 is as narrow as you could go on a 10.5" rim to have wheel protection, etc. I know 275's will fit on a 10" rim but it will leave little rim protection if at all, so a 285 would be very tight on a 10.5" rim. I'd look at 295's at least!
305 or 315. Don't skimp on rubber - thems all that keeps you from sliding into a ditch, off the track, or into the back of a car that suddenly brakes in front of you.