You are not going to get an 11" tire on a 12" rim....not without breaking something...beleive me we tried everything one day to get a buddy out of a jam breaking a tire machine in the process.

If you try to put a 12" tire on an 11" rim you will never get it to seal and it will constantly come off the bead.

With regard to tire width, you'll have less trouble with regards to width, a narrow tire on a wide wheel will stretch to fit (alot of the Volkswagon guys do this) looks weird nad leaves the bead open to damage, but can get you out of a jam. I've also seen people go the other way as well, My bike had 10" wide tires stock in the rear and I've run 12" wide blackwaters with no trouble, my Uncle went from 10" wide rears to 14" wide rear spider tracks with no trouble.

 

ITP Make 25" Mudlites for 11" rims 25x10x11 Mudlites for $60 You wouldn't need better tires than those and you can keep your rims.