depends on the terrain, is my rule of thumb. If you run lower pressure you risk more sidewall dammage (Im sure its not terribly a lot more, but eh...). If the trail your doing has sudden hard changes (jagged rock, or a ledge you might hit at any speed), I'd keep about 15-18 for good measure. If your running only sand, maybe 10-15. It really depends on too many factors, if its really grippy terrain, and you run low pressure, I've seen some people spin the wheels with the tires not moving. I've also seen people air down to about 8 PSI, doing a small hill climb, hit a small rock (4"?) which managed to pinch the tire (tire seemed un-dammaged) but managed to bend the steel rim, enough to leak air. He hammererd it back in and all was good, but... you know, just letting you know low air pressure can have that effect.