No sooner had we aired the tires down to 15 psi to drive to the campsite on the sand, when the forest service pulled up alongside and informed me I needed an ATV sticker for my pickup to go any further (off-road) ...  so we drove back out on the pavement with 15 psi in the tires and bought the sticker at the ATV shop next to the highway.  Leaving the next day, our little 12v compressor took about 5 minutes per tire to air back up to 35 psi, which was good enough to get us home.