What air pressure to you guys run on the BFG KM2s on a stock 18" rim?

 

What size?
I'm running 35x12.50 on stock 18's. The shop put 40 psi in, I lowered them to 35 and they feel a little better. But I've only put a few hundred miles on them so far, need to experiment with them a bit more.

 

They are 35x12.5s

 

Those are a load range D with a max of 50. 38-40 should be just fine. If you go lower, don't go any lower than about 36.

 

I'd stay in the 35 to 40 psi range for daily driving since you have them mounted on a narrow wheel.

 

They are plenty stable at 35, ride pretty smooth, and seem to have a slightly better contact patch than they did with 40 psi (judging by wet tire tracks in the garage).
I tend to run a little on the lower side on a narrow wheel so the center doesn't wear down as bad.
But like Wandell said, 35-40 range seems about right on the street.