PSI is set to your vehicles weight. The tires numbers say what they can handle as a max, if you go above the vehicles set rate, then you will start to bald the center tred of your tires, if you go below your vehicles set rate, then you will bald your tires tred on the edges of the tire. Most Xterras will be around 35-40 PSI, unless you really add to your vehicles curb weight by doing mods. If you have questions most of the service guys at tire places will be happy to explain it to you, thats how I learned

:clown: Oh, and you can find your recommended setting on the panel inside your driver side door jamb. hope this clears it up fort you a bit. :cheers: