i just got a new 400ex stock rim and tires on it r 20x10x9 i was wondering if 20x11x9 tires will fit on the stock rim

 

These should not be a problem. In fact...in most situations you can get away with an even larger tire "width" difference. For example...Turf Tamer 18x11x8 tires and 18x9.5x8 tires both are mounted on the same 8x8 wheel. As long as the rim diameter size of the tires are the same, which are usually the last set of numbers in the size description, you should be ok.