I have a dvd that came with my gen1 Raptor entitled "off-roading in the Raptor". In this dvd they discuss airing down and ford does not recommend going below 28 psi when doing "high speed" off roading. That was the only time a specific psi was mentioned. The rest of the time airing up and down was used in general terms. It does mention going 100 mph over rough terrain though.