what's the biggest tire I can fit on an 8.5" wide rim?

[blksgZ28]

I ran into a bit of a predicament, I picked up some new wheels($600 for wheels and tires) but I'm going to have to get some new tires pretty soon as the ones the wheels came with are a bit worn. The wheels sizes are 17x9.5 in back, and 17x8.5 in front. Some time down the road I'd like to get some lighter, slightly wider rims(17x9.5 all the way around, or 17x10 all the way around) but for now, I am stuck with these.

Any suggestions?

[Dr.Mudge]

255s would fit great on an 8.5", which was OEM with the C4s equiped with 8.5" wheels.