an 11.5" tire on a 6" wheel is just reatrded and will lead to premature wear. The lowest i would go on an 11.5 is an 8" wheel. Ive seen a 12.5 on an 8" wheel and it caused premature side wall cracking.
from what i was told you should not go less than 5 inches from the width of the tire so that would be 7.5. I have 31x10.50 and want to get some 33s but i have a 7 wide rim ill have to get a wider rim.