255/85x16's are allowed to be fitted on 6" wide rims....thou with the very positive offset I suspect there'd be some scrubbing on swaybar and chassis on full lock.
I run 225/75 R16 ATs. I know it's not the biggest as above but it's bigger than standard. On a side note i was in at Tyrepower researching my next set of tyres and they told me 225 was the widest legal fit on my standard SR rims.
"of all the things I've lost I miss my mind the most"