What's the widest tire you've seen on 9" wheel?

The widest I would go is 275 on a 9x17in rim.

The only possible benni might be you can run lower tire press. if its a drag radial at the track as the rim will want to put itself into the rim more. 4x4 guys will sometimes do this to keep the tire seated at low psi...run say a 12.5 wide tire on a 8in (wide) rim instead of a 10in rim.
 
I think with an 18in rim 285 is close to the 275 in width. I know many use 285's on 18's...315's may work but probably not the best. Contact patch and sidewall is what helps hook...with the larger rims size and lower profile tires on 17-18in rims at those widths the sidewall is already shot.