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Assuming the vehicle runs well and doesn't need a tuneup, or have a worn out engine, either you are driving it aggressively or making a lot of short trips, I would guess.

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How much time does the truck sit idle at traffic lights how much throttle it takes to get over 4,000+ pounds moving? Areodynamic that truck is not so faster you go the more fuel it uses. As temps cool off the truck uses more fuel during engine warm up. Several things around 100k are timing belt waterpump thermostat spark plugs wires cap and rotor and replace those leaking valve cover gaskets. The only real way to get gas mileage is ajust tire pressures to around 35 psi remove any extra weight from truck,take objects from roof rack waxed body work, drive with less throttle keep speed to under 65 miles an hour and avoid stop lights and in town driving. 4X4 trucks have extra drive train weight so they will use more fuel too.

13 mpg in the city or on highway??? if you're talking city miles then thats about right for an xterra..my 04 gets 13 in town and about 17 on the highway

Do you drive with a lead foot?

no seriously, thats the biggest factor in fuel efficiency.

other things like air conditioning, windows down, flatish tyres and heavy objects in the car all suck away you fuel.

Its a nissan. I have a nissan pathfinder. What i have learned about nissans is that they care about speed and not fuel. And its an older nissan meaning it doesnt run like the day it was made.