Tire rating is one thing, vehicle specs are another. Since same tire can fit many other vehicles pressures will vary. Heavier vehicles will need higher pressure than lighter vehicles.

The Xterra does not weigh all that much comapred to full size trucks out there.

What you should look at is the plackard found inside of the glove compartment that states the proper tire pressure. I believe it states something along the lines of 28 lbs. or so.

Running too high of a pressure (42 is way high) will result in uneven and premature tire wear. Your tires will start to wear on the inside mostly instead of even wear. Traction is also sacrificed and chances of hydroplaning are higher if the tires are overinflated (or underinflated for that matter).

If you do mostly highway then it's o.k. to set the pressure a little bit above the rating indicated on the plackard but not by much.

When I was running stock tires I found out that around 30 psi in the front and about 29 or so in the rear worked quite well.

If you have time the best way to test the ultimate pressure is the chalk test. You apply chalk to section of the tire drive a few feet over a dark surface like a freshly coated asphalt. By looking at the tire and the mark on the ground you can see the contact patch and that way you can determine whether the pressure is too high or too low. Talk to TJ on here on more details regarding the mentioned test.