255 on a 8.5 wheel you're not going to have good turn-in. The tire is going to want to skirm on the wheel and you're ultimately going to delay response of the tire to achieve max grip.
This is an AWESOME calculator, take some time playing around with it:
http://bndtechsource.ucoz.com/index/...alculator/0-20
I run 245/40 on a 8.5 wheel. I'd only go 255 on a 9" wheel IMO. You don't gain much going up tires on a 8.5 when you're at 245.