Verified answer

Actually, that's not totally out of line for a V6 4WD Xterra of that vintage if you do alot of in town driving. If you have a V6, 4WD, Automatic. It's only rated for 14 MPG city and 17 MPG highway. If you're getting 13 MPG and you do primarily city driving then that's pretty close to what you can reasonably expect. If you do mostly highway driving, then it's a bit low. You may just want to have tune up if you're close to the 100k mile mark. You might want to consider having the timing belt done as it's scheduled to be replaced at the 105k mile mark.

Might need an oil change, tune-up (spark plugs, wires, new fluids etc) or over/under-inflated tires. You might need a new air filter. Even a driving habit could be causing it.

Personally I would check the tires and air filter first, as over or under-inflated tires will cause premature tire wear, and the air filter is just so darned easy to check :p

Hope this helps!